The two most important sects in Islam are the Sunni and the Shi'a. The split between these two sects dates back almost to the very beginning of Islam.
At the death of the Prophet Muhammad, the two sects developed over the issue of who would succeed the Prophet. The Sunni decided to follow Abu Bakr. They felt that the leader of Islam should be the person most qualified to lead. The Shi'a chose to follow Muhammad's son in law, Ali. They felt that the leader of Islam should be should come from within the Prophet's family. This led to the creation of what are now rather different sects of Islam.
Sunni and Shi'a share the same basic beliefs that make them both Muslim. However, they disagree on a number of details. Perhaps most important is the disagreement over sources of religious authority. Shi'a believe that there should be individual leaders (Imams) whose authority comes directly from God. They venerate past Imams as saints. Sunni do not believe this.
There are other, much smaller sects as well as splits within the Sunni and the Shi'a. However, these are the major divisions in Islam.