I read this thread a bit late, but insha' Allah my response will answer your questions.
When Islam was in the early years, the Quran was translated by Gabriel the angel
to prophet Mohammad(pbuh) who taught the early sahaba the noble Quran, the right way of reading, the meaning, and the rules muslims should abide to. The sahaba(his companions,early muslims) wrote the hadeeth, which is the translation of the Quran and rules of Islam, and those hadeeths reached to us today by the effort of very pious scholars who lived all their lives travelling from one country to another to collect them, and they were very accurate and specific in approving them, there is a science in Islam called the science of hadith, it shows u which is a false one and which is true, they use a method called Tawater(which means-recurrent- the hadith must be narrated from one companion to another until it ends up to prophet Muhammad(pbuh), and these companions must be trustworthy and known of their piety). So this is what the true muslim should follow, the sunnah of prophet Mohammad and his companions.
Later after his death, there were the 4 khalifas after him, on the time of the third khalifa Othman (may Allah be pleased with him there was a big affliction that happened that ended by murdering the khalifa by the <b>KHAWAREJ</b> these were arrised by
one man who is originaly jewish who claimed to be muslim his name was Abdullah bin Salam, they killed the khalifa and they were seperated into 3 parties each wants one as the khalifa, these khawarej are what are known nowadays as shie'ts, I will not write their beleif now and doctrine it will take such a long post, but in short they beleive that Ali(may Allah be pleased with him) who was the fourth khalifa and the cousin of prophet Muhammad should have been the first khalifa and many other things which is wrong((at the end they turned their back at him after they made a big affliction between him and our mother Aisha(may Allah be pleased with her) and another one between him and Mua'wiya(the khalifa after Ali was killed)), the hadith they use is not recurrent, the tafseer they use is so imaginary and so on, and after that there were many other doctrines that are totally not Islamic in any way, like the qadyania(9this one their leader claims to be a prophet:rolleyes:.)) , very sad for him, and ahmadiya and drooz and many others I dont recall them now, and suffism as well, in sufism at the end they reach to athiesm( may God help Us). all those who changed they changed out of their own writings, forged stories about the prophet and his companions, which can be easily recognized if u study the science of hadith.
I have written here of my little knowledge trying to make it simple, may Allah accept from me.
If u wish to learn more go to these sites
http://www.uga.edu/islam/home.html
above u will find about different doctrines I metioned, and many other sites that are trustworthy of seeing.
http://www.islamic-awareness.org/
http://www.islamic-awareness.org/Hadith/
http://www.usc.edu/dept/MSA/fundamentals/hadithsunnah/
http://abujasir.tripod.com/hadith.htm
I hope these will be sufficient, u will read also more about true Islam, and u will find many answers to your questions insha' Allah (by God's will).
Sorry for my long post, I hope i answered your question, if u find anything that is not clear please inform me Insha'Allah I will try to make it clear .
If I am right it is from Allah, if I am wrong it is my mistake and Ignorance.