Islam is an Abrahamic religion which follows the teachings of the prophet known as Muhammad, who lived from 570 to 632 AD. Islam is most prominant in the Middle East, Turkey, Central Asia and North Africa. A follower of Islam is known as a Muslim.
Unlike Christianity (but like Judaism) Islam has an active Legal component known as Sharia, based around the muslim holy book (the Koran), various sayings of the prophet (the Hadith) and accumulated judgements by various islamic jurists to fill in gaps (Ijtihad).
There are two main strains of Islam, based around the line of succession after Muhammed's death.
There are other sub-sects based on differing interpretations of Islamic law.
While many Muslims blend in effortlessly and peacefully with other cultures and faiths and there have been many instances in which Muslim-dominated countries peacefully interacted with non-Muslim countries, not all are so eager to coexist. A significant number of Muslims hold a hatred for "Western" culture and religion, and they organize to express this hatred in excessively violent ways. The most well-known example of this is the terrorist group Al Qaeda, responsible for the September 11 airplane hijackings and suicide bombings that cost thousands of civilian lives. These activities are not always directed outside of the Islamic world and will often be between sects.