Hi Marb
Welcome to CR. I can only tell you what I understand about the sects within Islam. Please remember that any links I give to a sects websites may be seen as one sided.
The 2 largest groups within Islam are Sunni and Shi'a (with Shi'a having between 10-15% of the followers of Sunni). The split between the 2 groups was simply due to arguing over who should lead the followers after the death of the Prophet Mohammad (pbuh). See here for the Sunni/Shi'a split
The Origins of the Sunni/Shia split in Islam
This is a good short article about the split, which tells it as it is
What's the Difference Between Shia and Sunni Muslims?
Most sects, as you will see, are subsects of Shi'a Islam. Sufi is the third largest sect (not from Shi'a) and is the mystical side of Islam.
Sunni - within Sunni Islam there are 4 madhabs (schools of thought) and they do not always agree.
sunni islam islamic studies general resources
Madh'hab - Wikipedia, the free encyclopedia
Shi'a (or Shi'ite) - this is where most of the sects have come from. Iran is a Shi'a State.
Shia Islam - Wikipedia, the free encyclopedia
Sufi - The mystics. They are much less concerned with politics and dogma. The truth be told, I mentally lean toward Sufism.
What is a Sufi?
Kahrijite - A subsect of Shi'a. In modern times we would call them Democrats.
Kharijites: Early Muslim rebels espoused democratic principles
Wahhabi - In my view, very conservative extremists and believe all other Muslims have lost the true path (of course most sects say the same thing). I am not a fan, if I just say they are the highly political ruling sect of Saudi Arabia (where it is illegal for women to drive and the religious police a few years ago killed schoolgirls trying to leave a burning school, because they were improperly dressed- better that they die than show their face and hands in public
) you may get the idea. They used to be confined to Saudi but when the money from oil started rolling in they began supporting groups outside their own country (look at Afghanistan, this was set up by Saudi). I do agree with some of their issues (eg not idolising the Prophet Mohammad (pbuh) and not following religious leaders blindly), unfortunately they ruin their good works with their open teaching of hatred to non Muslims (and most Muslims too) and absolute oppression of women. Despite the title, this is quite an unbiased look at who the Wahhabi's are and what they want
Wahhabi
Ismaili, Zaidi, Fatimid and Nizari (or Assassins) - are all subsects of Shi'a.
History of THE ISMAILIS
Zaidiyyah - Wikipedia, the free encyclopedia
The Agha Khan's flock
Druze - they have some lovely principles but again are not considered Muslim, by the vast majority of Muslims, as they believe G-d became incarnate in al-Hakim (a religious leader).
Druze
There are also
Ahmadi (the member called inhumanity is Ahmadi and can tell you about their sect) but I did find this site
AAIIL: Ahmadiyya Anjuman Isha'at-e-Islam Lahore (Lahore Ahmadiyya Movement for the Propagation of Islam)
and
Baha'i - CR has a board for followers of the Baha'i faith, descended from Islam but quite different in beliefs. Have a look at their board they have some great threads.
Geography has a lot to do with which sect a born Muslim belongs to. Hope this helps you and thank you for your interest in our various religious beliefs.
Salaam