O.K. so now I thought I'd talk about the various offshoots of mainstream Islam (yes, there are a few).
First of all, I am a Sunni (aka Orthodox) muslim. As the mainstream of Islam, this accounts for about 80% of the worldwide muslim population. Our first, and most important text, is the Quran (or Koran). This was dictated directly to the Prophet Muhammed {pbuh} via the angel Jibril (Gabriel). It consists of 114 chapters (surah's) each classified as to whether they were revealed in Mecca or Medinah. In addition to this, there is a body of sayings and doings of the prophet {pbuh} known as the Hadith. These are not the direct word of God and hence may have been transcribed/heard incorrectly. This is why they are classified as Sahih (authentic), Hasan (fair), Da'if (weak), and Saqim (infirm). After the Quran, we mainly try to live by the Sahih Hadith's. As a result, Islam needs no clergy/priesthood to interpret the words of God/the Prophet...they're written in plain language anyway. Imams are not priests/monks but teachers (think of it like having Chemistry or Maths teachers...imams are Islamic teachers who fulfil duties such as conducting prayers, weddings etc...). According to Islam, faith is between YOU and GOD with no one intervening (i.e, you only pray to God and no one else).
When the Prophet {pbuh} died, as he had no sons, there was no clear line of succession. As a result, the later leaders of the state (Caliphs: Abu Bakr, Omar, Uthman, Ali, Muawiya...) were not recognised by certain muslims. These were later to become the Shia sect who did not recognise the first three caliphs, and honour Ali (the Prophet's {pbuh} son-in-law). They claim to trace their descent from Ali's first or second son so calling themselves Sharif or Sayid respectfully. The other main difference between Sunni's and Shia's is that some of the latter may not recognise certain Hadith. Also, Shia's have a priesthood (mullahs/ayatollahs) which is why edicts, fatwahs, given by their leaders are hard to rescind, it's similar to a Catholic disobeying the Pope.
There are further ofshoots of Shia's yet again. I must stress that the majority of Shia's have beliefs almost identical to Sunni Islam. However, a small minority may also take a more extreme view and go as far as revering Ali, whilst very few may believe certain ayatollah's are closer to God and so on a par with the Prophet {pbuh}. There are also some who firmly believe in magical Sufi mysticism (known as mumbo-jumbo in orthodox Islam - this is a type of philosophical ideology that is a bit like a fusion of Islamic thought with Buddhism). There are even Sunni's who dabble in "grave worship", something seriously forbidden in Islam. Again these people do not conform to the mainstream of Sunni/Shia Islam, but back to the other sects...
The majority of Shia's recognise a succession of 12 Imams, but a small group of them, reject the seventh. This sect is known as the Ismaili sect. A further offshoot of these (the Nizari's) dispute exactly who the seventh Imam was and say that Aga Kahn is a direct descendant and the saviour of mankind (even though this man spends lots of money gambling at the horses, something which orthodox Islam forbids).
And so on and so on...there are many minor sects of Islam today. It will take a lot of time to go into all of them so instead I will now point out some definite non-muslims that call themselves so but do not adhere to some basic tenets of the religion.
The Ahmadi's believe that a bloke called Gulam Ahmed is the reincarnation of Jesus {pbuh}, the Promised Messiah, the saviour at the end of mankind (mahdi), and Krishna (quite multi-talented eh?). Firstly, muslims do not believe in reincarnation and secondly, the only thing in Islam that you can't be forgiven for is to believe in other gods besides the one true God (i.e. no polytheism) and Krishna is one God manifested as a pantheon of Hindu gods (at it's minimum, part of the Trimurti, but this is still defined as polythiesm in Islam). Ahmadi's also believe that Jesus {pbuh} didn't die but went to live in a village in Srinigar!
This sect has split into two groups, the Qadiyanis who say that Gulam was also a prophet (despite Muhammed {pbuh} being the last according to Islam), and the Lahories who say that he was a renewer of faith.
Another non-muslim group are the Nation of Islam or black muslims of the U.S.A. (good ol' US of A). These people are not muslims because they are basically racist (again, this is strictly prohibited in Islam). They believe that white people are the offspring of devils genetically engineered by a mad scientist on an island somewhere thousands of years ago (I mean come on, be serious). Islam states that it doesn't matter whether you're black, white, brown, orange, purple, blue, green... a muslim is a muslim and an equal. They also believe that Elijah Muhammed (their leader) was a prophet who shared God's powers. This they ascribe to a man who had several illicit affairs with his followers and either forced them to have abortions or dumped them once the children were born. They further believe that God lived on Earth in the 60's (in San Francisco no less...I mean of all the places to choose, why there?) and was the 'spirit' of a man called Wallace. No I'm not making this up, these people seriously have some screws loose.
It's interesting to note that despite being thought of as prominent black muslims, both Malcolm X 
and Muhammed Ali later renounced this racist doctrine as against the spirit of true Islam and in so doing, converted to Sunni Islam towards the end of their careers.
There are other (muslim and non-muslim) sects such as Zikris and Nurbakshi's...again...etc, etc...
Why make such a fuss you say? The reason is that these latter sects proclaim theirs is the only Islamic religion despite not conforming to even the basic foundations of Islam, and then I get grief from people wanting to know "Is that what Islam's really like?". The answer is no, Islam is a religion of peace and submission to God (not submission to some wierdo with grandiose delusions).
I hope that's cleared some misconceptions up (if i've got something wrong on these pages, please feel free to e-mail me with specifics so that I can correct any mistakes).
2nd June 1998