It's hard to answer the Sunni/Shia question without getting into a whole bunch of religious and political history, the ethnic divisions in the Near East and many, many other details.
The simple version goes sort of like this...
After the death of Mohammed (pbuh) Islam continued to spread. There were differences of opinion about who was supposed to be in charge of the Faith. Was it the sayeeds (the descendents of the Prophet)? If so, which ones? Was it one or the other of his close friends?
It devolved into the usual sort of thing - ethnic divisions, "the last argument of kings", and a certain degree of factionalism. As my good friend Mushtaq Ali Al-Ansari - martial artist, musician and poor wandering dervish (semi-retired) puts it:
Re: What's the difference?
It's really quite simple;
The Sunnis make up the largest Muslim faction. You can tell them because they hate the Shia over some events that happened about 1400 years ago, and because they are of the opinion that God is running a democracy and that if a majority of them think that things should be done a certain way then God has to agree.
You can tell A Sunni sometimes because of the habit that some of them have of cursing Ali ibn Abu Talib, the son-in-law and cousin of the Prophet, who the Prophet raised from childhood and married to his most beloved daughter.
The Shia are a much smaller faction of Muslims. They hate Sunnis because of events that happened about 1400 years ago. They think that certain people are more capable of knowing what is best for you than you are yourself. They know who these people are because they all agree that this person or that person is one of these special people.
You can sometimes tell Shia because some of them delight in cursing Abu Bakr, the Prophets closest friend, who the Prophet loved very deeply.
Now the Wahhabis have solved the problem of who to hate by just hating everyone who is not them and desiding that anyone who does not agree with everything they say is not really a Muslim and should be killed.
You can sometimes tell a wahabbi by the habit that some of them have of sitting around making lists of every possible action that invalidates someone else's Islam.
All three groups agree on almost nothing,.........
Except that Sufis are all innovators and heretics of the first water and should probably all be killed because most Sufis don't have anyone to hate and are much more concerned with their own Islam than that of their neighbor's.
I hope that this helps clear things up a little.
Now I'll remove tongue slightly from cheek.
The Sunni are the largest division within Islam. The Shia are the next largest. The Sunni believe in one line of succession. The Shia believe in another and feel that that line included a number of, not prophets, but specially chosen leaders known as the Imams. All of them agree on the important parts of Islam. The similarities are much greater than the differences. There are other smaller sects such as the Alawaites who are important in Syria and some odd splinter groups like the Ahmaddiya. Don't worry about that too much.
The Wahabis are the followers of Muhammed ibn Abd Al Wahab, an eighteenth century man from the Arabian Peninsula. He preached a very strict, literal and rigid form of Sunni Islam. It became an important political movement with the ascendance of the House of Saud in the 1920s because it gave them control of Mecca and therefore the Hajj. It swung into high gear with the discovery of oil. Most Wahabites claim there is no such thing. They are just Muslims, the real Muslims. At most they will call themselves "Salafists" or original Muslims.
Sufism is a tradition within Islam. More Shia than Sunni are Sufis, although Sunni Turkey is the home of many, many Sufis. Many Sufis trace their lineage back to Ali (may G-d be pleased with him) who is disliked by many Sunnis as we see above. Sufism stresses the importance of a personal relationship with G-d, constant remembrance of the Divine, and a close relationship with teachers and mentors. It tends to be anti-authoritarian and the repository of the mystical and transcendent within Islam. As such, it is hated and forbidden by rulers. Being a Sufi is illegal in every country whose legal system is based on Sharia.
Sufis are found all over the place. Disproportionate numbers of them are scholars, artists of various sorts, and the poor. Sometimes all at the same time
The Turkish Janissaries were tutored by members of the Bektashi Sufi order. The Whirling Dervishes are Sufis of the Mevlevi order founded by the famous poet Rumi.
While Sufism is a current within Islam there is an adage which applies "Mystics tend to recognize one another. Fundamentalists see only themselves." Sufis have generally gotten along with similar people from other religions. The great Rabbi Moses Maimonedes and his son were both extremely pious and observant Jews. They were also members of Sufi groups and are recognized as Shaykhs by most orders. The great Kabbalist known as "The Ari" had Sufi and yogic training. The religion of the Sikhs has strong Sufi roots and a similar outlook to the best of my meager knowledge. In fact, my Shaykh is one of those who believes that the Sikhs really are Sufis and are only something else because of stupidity and prejudice on the part of certain Muslim invaders of the subcontinent.
Heck, that quote bears some repeating. I'll say it again....
"Mystics tend to recognize one another. Fundamentalists see only themselves."
