Different groups. It's like the difference between Catholicism, Eastern Orthodox, and the various Protestantisms.

Sunni and Shia divide over who is supposed to run the "church" after Mohammad's death. Kind of like if there were two popes. Ismaili is a break off of the Shia(I believe) but I could be wrong about that last one.

After Muhammad died Islam split over WHO was the next successor. Sunni believe that the muslim community should choose the successor. Shi'ah believe that the successor was divinely chosen by Muhammad himself. Ismalli is a further subcategory of Shi'ah. This schism process is pretty regular in religion because despite everyone believing in a particular faith, they never agree on the details. See The Reformation. See Samaritans. See Zen.

the same core belif they divided over who should lead after Mohammed Sunni belive the Abo baker should lead Shia belive Ali should lead I don't know a lot of details although i'm muslim :) what the important for me is that i follow Mohammed path (quran and Sunnah)

The word “Sunni” itself comes from the term “Sunnah”, which means the teachings of Prophet Muhammad, for they are strict in abiding by these teachings without any introductions, interpolations, or omissions. The "Sunnees" (Sunni) constitute more than 90 percent of the Muslims population. The 'shia' and 'ismaili' (which is another sect of shia) are two of the earliest groups to deviate from the path of the sunnees (those who followed the way of the Prophet Muhammad (peace be upon him)). Their innovations include revilement of the very Companions of the Prophet Muhammad (peace be upon him), rejection of his successors and dispisement of his wives. Their teachings wander far away from mainstrain, moderate ture Islam. Therefore, the 'shia' and 'ismaili' are man-made sects which are really not following the original true teachings of the message of prophet Muhammad (peace be upon him). You can compare them with other false man-made sects like 'the Nation of Islam' in USA and 'the Bahaism'. For detailed information on this subject please visit the following link: "Sects Attributed to Islam" http://www.islamreligion.com/category/86/

1) "Islam consists of a number of religious denominations that are essentially similar in belief but which have significant theological and legal differences. The primary division is between the Sunni and the Shi'a, with Sufism generally considered to be a mystical inflection of Islam rather than a distinct school. According to most sources, approximately 85% of the world's Muslims are Sunni, 14% are Shi'a with the 1% beiing other various small minorities and Islamic sects. - Sunni: Main article: Sunni Islam Sunni Muslims are the largest group in Islam hence the title 'Ahl as-Sunnah wa’l-Jamā‘ah' (people of the principle and majority). In Arabic, as-Sunnah literally means "principle" or "path". The Sunnah (the example of Muhammad's life) as recorded in the Qur'an and the hadith is the main pillar of Sunni doctrine. Sunnis believe that the first four caliphs were the rightful successors to Muhammad; since God did not specify any particular leaders to succeed him, those leaders had to be elected. Sunnis recognize four major madh'habs (schools of thought): Hanafi, Maliki, Shafi'i, and Hanbali. All four accept the validity of the others and a Muslim may choose any one that he or she finds agreeable. There are other Islamic sects that may be considered as being Sunni yet are believed to have departed from the majority by introducing bidah (innovations) and extreme political views which are divorced from Islam. There are also several orthodox theological or philosophical traditions within Sunnism such as the recent Salafi movement, which sees itself as restorationist and claims to derive its teachings from the original sources of Islam by refuting the established schools of thought. - Shi'a: Main article: Shia Islam See also: Succession to Muhammad The Shi'a, who constitute the second-largest branch of Islam, believe in the political and religious leadership of Imams from the progeny of Ali ibn Abi Talib, who according to most Shi'a are in a state of ismah, meaning infallibility. They believe that Ali ibn Abi Talib, as the cousin and son-in-law of Muhammad, was his rightful successor, and they call him the first Imam (leader), rejecting the legitimacy of the previous Muslim caliphs. To most Shi'a, an Imam rules by right of divine appointment and holds "absolute spiritual authority" among Muslims, having final say in matters of doctrine and revelation. Shi'a Islam has several branches, the largest of which is the Twelvers (iṯnāʿašariyya) which the label Shi'a generally refers to. Although the Twelver Shi'a share many core practices with the Sunni, the two branches disagree over the proper importance and validity of specific collections of hadith. The Twelver Shi'a follow a legal tradition called Ja'fari jurisprudence. Other smaller groups include the Ismaili and Zaidi, who differ from Twelvers in both their line of successors and theological beliefs." Source and further information: http://en.wikipedia.org/wiki/Islam#Denominations Further information: http://en.wikipedia.org/wiki/Islamic_schools_and_branches http://en.wikipedia.org/wiki/Sunni_Islam http://en.wikipedia.org/wiki/Shia_Islam 2) "Ismāʿīlism (Arabic: الإسماعيليون al-IsmāʿīliyyÅ«n; Persian: اسماعیلیان Esmāʿiliyān; Urdu: إسماعیلی IsmāʿīlÄ«) is a branch of the Islamic faith. It is the second largest part of the ShÄ«‘ah community, after the mainstream Twelvers (Ithnāʿashariyya). The IsmāʿīlÄ« get their name from their acceptance of Ismāʿīl ibn JaÊ¿far as the divinely appointed spiritual successor (Imām) to JaÊ¿far aá¹£-Ṣādiq, wherein they differ from the Twelvers, who accept MÅ«sà al-Kāżim, younger brother of Ismāʿīl, as the true Imām." Source and further information: http://en.wikipedia.org/wiki/Ismailism