To say "There is no God but Allah and Muhammad is His Messenger" and to do the 5 basic pillars. To follow Islamic law (Fiqh), which is the key here. It's kind of like Shariah... but for the individual person. This isn't just limited to the 5 pillars, but all personal Islamic laws (basically, all the literal injunctions in Qur'an as illustrated by Prophet(saw)'s example).
Sources: For Sunnis (orthodox Muslims, who follow Islam through the Prophet(saw)'s example), it's from living jurists who follow one of the four Sunni schools (Madhabs) of Law (Fiqh): the Hanafi, Maliki, Shafi‘i, and Hanbali schools.
To actually believe in God (Tawhid), the Unseen, Revelation, Prophethood, Day of Judgement, and Destiny (Qadar, whether good or evil). Wikipedia: "literally meaning "to learn", "to fully observe one's faith" or "to learn one's faith", and lexically meaning affirmation and confirmation in the heart."
- Tawheed is the key here. It means truly BELIEVING in the Oneness of Allah. The best way to understand that is to understand what that is NOT. Which is shirk. Any and all shirk (even all the little shirk that we're preached regarding in all those sermons/lectures/etc.) will negatively effect Tawheed, even NEGATE it. Where does shirk start? With intention (Niyat). There's of course the big shirks, worshipping other false gods, assigning Godlike traits to humans (distinguishing between saying someone has an attribute of God vs. someone REFLECTING an attribute of God, the latter is an alright, higher level of understanding). But the other shirks are summarized easily in this: Mahmud ibn Lubayd also said, "The Prophet (peace be upon him) came out and announced, 'O people, beware of secret Shirk!' The people asked, 'O messenger of God, what is secret Shirk?' He replied, 'When a man gets up to pray and strives to beautify his prayer because people are looking at him; that is secret Shirk." You get the gist of it. Technically speaking, athiests do have a god... themselves. So avoid athiestic thought, which is so easy to slip into (not remembering God when doing something, then you do it for yourself, and start getting into dangerous shirk territory). Even with acceptable Tawheed, having some Self-Centered thought left behind will later on block any further spiritual progress beyond this point. Do not think that you are safe from shirk.
- Anyway, Iman contains all the articles of belief within the religion, in addition to all the laws/injunctions/etc. from the first part, Islam. This is why society becomes a factor and everyone gets stuck here. If you cannot apply even the Sacred Law (Islam, or Fiqh) to yourself, then you flirt your way into Iman, you can't really complete Iman. And even if you do manage to somehow start embodying most of the basics of Islamic law sufficiently for the soul (I'm using ambiguous terms, but really... the rule of thumb is to try for 100%), you have to then believe in the things here which is a civilizational conflict. To believe in God, Prophets, Heaven/Hell/etc. goes against the philosophical underpinnings and principles of Western civilization. So not only must you embody a life adhering to alien 'laws' (personal laws, for yourself), you then have to change your perception of the world from everyone else. This is so difficult, not just literally in Western civilization but anywhere on the globe right now. To hold onto your Iman right now, is to try and hold onto that ball of fire with your hand.
- And there are differences in Aqeedah! For instance, that popular movement in Pakistan, 'Jamaat-e-Islami', based on the teachings of Maulana Maududi is HUGE. It's big also in India, Bangladesh, and Sri Lanka. It won 11% of the popular vote in Pakistan and and has 53/272 people in the Parliament thing there. But guess what? It's been excommunicated (in fatwa) from Sunni Islam by senior Hanafi/Maliki/Shafi'i/Hanbali clerics. Why? Maududi's teachings violate basic principles of Aqeedah. To believe in the incorrect ideas regarding Heaven/Hell and all these other things that don't even concern us yet, WILL have an effect on your Iman, which DOES concern us right now. With deviant or perverted Iman, you can't even move onto Ihsan, you'll wind up locking yourself into some kind of zealous, dogmatic, mindset if you try to press on (deviant spirituality). That's dangerous. And how many of us even know what basic Aqeedah even is, let alone some of these differences between other people? Lawlz. I was lucky, my mom's family is strictly Hanafi and knowledgeable about it so my mom raised me with the basic principles correct according to that school (Deobandi Hanafis follow Maturidi school).
Sources: Varies. Most all current Sunni movements follow the Ash'ari and Maturidi schools of 'Aqeedah, but there were plenty of older ones that came and went that I personally like to read up on. It's mostly philosophical bickering/arguing.
Sometimes spelled 'Ahsan', or 'Ehsan'. It means "perfection" or "excellence" in religion. From Hadith: "[Ihsan is] to worship God as though you see Him, and if you cannot see Him, then indeed He sees you." This is made up of basically spirituality (Ruhaniyat), piety (Taqwa), and mysticism (Tasawuf). The key thing here is Tasawuf because it's a thing you do, while the others are things you achieve. They're all kind of related.
- Ihsan is often considered the inner dimension of Islam, while Shariah or Law (both personal and societal, Iman and Islam), are like the outer dimension. Says Imam Malik (ra): "He who practices Tasawwuf without learning Sacred Law corrupts his faith, while he who learns Sacred Law without practicing Tasawwuf corrupts himself. Only he who combines the two proves true." If you make it here, you have achieved respect for Islamic law (Islam), belief free of disbelief and shirk (Iman), and now this (Ihsan) is the way you purify your Self and achieve sincerity and purity of intentions and actions (because anybody can appear to be externally going through the motions, but God isn't fooled). Abolishing Self-worship. I like to say Self-centered or Selfishness, but these have such a wide scope of definitions but definitely the more extreme forms of Self-centeredness and Selfishness are the same as Self-worship or Self-glorification. The roots of this are actually in simple Psychology (ego and all that).
- There are movements like Tabligh-i-Jamaat which try to disseminate the expertise of these people through wide exposure. Many Sufi Shaykhs travel and give speeches on personal growth (Islahi Khutbat) and you can find such types of sermons/lectures all over the place online, especially from famous scholars of Fiqh (Maulanas, Muftis, etc. who also had spiritual education). Despite the fact this is the most esoteric part of Islam, the sheer potency of it demands that at least some of it be used effectively on people to help them, in line with Ruhaniyat. You'll see people like Hamza Yusuf (I think he's the most popular Sufi in America atm) give essentially watered down, precision-targeted bits of information to the youth in an effort to jump-start them when they have dead batteries.
- Tasawuf: The definition of Tasawuf according to Al-Ghazali is to purify the heart and dedicate it to Allah without turning your attention to other than Allah. It is simply expressing spiritual faith in physical practice. This is basic Sufism. Dhikr, fasting, etc. Says Imam Al-Junayd, "We did not acquire this level of Tasawwuf just by talking about it; but rather, we acquired it through spending the nights awake in prayer, experiencing hunger from fasting the days, and going against our worldly desires."
- Taqwa: "There are five signs of righteousness: a gentle disposition and a soft heart, shedding tears of regret, ascetism and not caring about the world, being unambitious, and having a conscience." -Shaqiq al-Balkhi (ra). What gets you to this? God-consciousness. This is a result of Tasawuf as well. Usually Taqwa is defined colloquially as simply fear-of-God, but that means essentially the same thing. If you have a conscious feeling of God being right there, as much as you love Him (and probably feel comforted in times of trouble), you'll be pretty scared. It's the next step up from thinking that you see Him or that He sees you. It's feeling it, because He really is there. Being this pious/righteous is very unnatural for a human being with his ego. A child will rarely if ever be an angel when his mom or dad isn't around. But if the mom or dad is terrifying enough (unlike children we know the implications of why we should fear something that much greater than us) and right there, he/she will behave completely differently. But God is ALWAYS there. So a strong sense of Taqwa can help push and cement a personality into this stage of piety/righteousness.
- Ruhaniyat: This isn't talked about much. Because people (even Sufis) rarely use a word to describe this. Since this is all about knowing/feeling, not words. The only official definitions I find around are 'soulfulness' or 'spiritualness'. This is basically like the intensity of the soul's manifestation. The Ruh in Islamic philosophy has several levels, this would be like the deepest levels manifesting themselves through the rest (which include the lowest, your physical makeup). When you get 'burning' inside... this is it. It's like spiritual awakening, your soul is almost literally waking up and manifesting itself. In Transpersonal Psychology, it's kind of like a 'Spiritual Emergence'. This can range in intensity, from simply a spark to outright MADNESS as if one were struck by a lightning bolt from the Heavens. Please note, this has absolutely nothing to do with normal kinds of 'madness', it just might be confused therewith by Western specialists in Psychology since they don't have terms to understand this idea outside of the Transpersonal arena. The best 'secular' definition I could find from Wikipedia: "The term "Spiritual Emergence" was coined by Stanislav and Christina Grof (1989) in order to describe a gradual unfoldment and appearance of psycho-spiritial categories in a persons life. In cases where this spiritual unfoldment is intensified beyond the control of the individual it might lead to a state of "Spiritual Emergency". A Spiritual Emergency might cause significant disruptions in psychological, social and occupational functioning, and many of the psychospiritual problems described above might be characterized as spiritual emergencies (Lukoff et.al, 1998). Besides the psychospiritual categories mentioned by Turner et.al (1995) and Lukoff et.al (1998), Whitney (1998) has also made an argument in favor of understanding mania as a form of spiritual emergency." That still doesn't really explain Ruhaniyat. =\ This has been known in Sufism as the phenomenon when a Shaykh or Master kind of 'spiritualizes' his Mureed or Apprentice. It's like in Rumi's poetry, where he describes the 'burning heart' of a Shaykh alighting the heart of a Mureed like you would alight one candle with another already lit candle. You can describe this burning as love too, pure love or pure faith. It's just raw 'soulfulness'... light. I described the common principle of this in the speeches/lectures/teachings of Sufi Shakyhs above. This is purely 'feeling'. When you hear a lecture by a good scholar about personal stuff (rather than Law) and you feel revitalized beyond what you normally would from a lecture. A revitalization that almost makes you look around and see things differently. That's like a very tiny spark of this. It can also happen by an act of God. Which can be low in intensity, or again... like a lightning bolt. In the latter case, 'Spiritual Emergency' is definitely more apt a description than simply Spiritual Emergence. A lightning bolt-like case would be like when after much meditation, Ibn Sina felt he figured out existence. He ran naked through the streets of Baghdad shouting 'I am not me. I am God.' (What he actually meant was 'I am not me. And everything that is, is God.') This is accompanied by 'knowing'. This distinguishes it from simple madness. Your Qalb (spiritual heart) is a source of knowing and intelligence. If it's real soul activity, the other parts of Ihsan, and even Iman and Islam are suddenly solidified in a strange way. You get the sincere intention to figure out Islam if you don't know already... in fact, you'll soon figure out where to look if you didn't already know (the four madhabs), and your Iman gets 'fixed up' in that you have clear solid beliefs in those things of 'Aqeedah. I'm describing the really intense, very very rare forms of this soulfulness here, but you can get tiny sparks of it which are not uncommon. And these can come at any stage of Islam/Iman/Ihsan. But if Islam and Iman are not perfected, they won't stick... at best the person might get a sudden awakening or englightened period where their Islam or Iman is amplified into correctness, but it will stall out and die until the foundations are perfected (thus the word, 'Ihsan', or perfection). When you get 'sparks' of it, you won't hesitate to figure out the Deen and implement it, you'll be like a person on a mission. But ya, to rehash... this will help solidify all the previous levels of the Deen before here. This really shouldn't have been given the most attention out of anything here, but this is so tough to explain. Another way to put it is that this is kind of like being 'in the zone' with regards to Deen (and other things). A believer who has achieved this properly and ultimately (usually some kind of Sufi with a spiritual guide), his perception of things is a bit different. They can trust their feelings because their feelings are coming from the soul (through the Qalb). It's like they look at a mass of garbled text (to represent random information one might come across), and suddenly the letters assemble into proper sentences (it's like the 'truth' self-manifests out of anything). It's knowing... from the inside. I'm only going on about this cuz it's such a lofty ideal to keep in mind as motivation at times.
Sources: The most 'root' sources are from living masters of one of the four main Sufi orders that have survived intact, the Qadiri, Naqshbandi, Chishti, and Suharwardi tariqas. This is not knowledge that can be contained in a book, this is only contained in breath and experience. At the very top level, this functions like Master/Apprentice (think Star Wars' Jedis...).
Addendum re: Ruhaniyat (BTW, let's not kid ourselves. You don't have this, probably nobody you know does. We get sparks of light as mystic emotion, that's it. Emotions live and die in the moment and anyone, Muslim or not, can get them. You're not going to run into a person who embodies this (Ruhaniyat), these types of people live a different lifestyle. And when you do (when they give speeches/sermons/whatever), you'll know that they know. You're sure of it. You can see it on their faces. If there's any doubt, then that person is probably just sputtering with sparks if anything. But when someone really has it... it's infectious. You'll know with certainty that they know everything else with certainty, but your knowing will only encompass that. But even then, you have to reach a certain level of basic Islam/Iman before you can even percieve that much, but most Muslims have sincere enough hearts to see the Truth when it's in front of them. Except youth of course... youth misled by the pretty dunya. )