"“Religion of Peace,” my ass; Islam is the religion of ignorance and hate."- PZ Myers [1]
Islam is another one of the three Abrahamic religions, along with Judaism and Christianity. Islam is a Monotheistic Faith which Muslims claim is the final revelation of Allah following the revelations to Adam, the Scrolls of Abraham, the Torah, the Psalms, and the Gospel.
Muslim beliefs do not include the Christian concept that Jesus was the Son of God or saviour although their Mythology has Jesus, or Issa, as an extremely important prophet. The holy text of Islam is the Qur'an, which they believe is the "revealed word of God" as transmitted by the Angel Gabriel through the prophet Muhammed who they claim was the final prophet sent to earth by God.
Becoming a Muslim
To become a Muslim you must say (and believe,) "There is no god but Allah and Mohammad is his prophet."
Making the declaration is easy. You can make the declaration before you understand what you're letting yourself in for. However, if you change your mind after you’ve said this, you're considered an apostate and apostates can face execution. See Fun:Becoming a beef monkey.
Afterlife
Muslims believe in Hell. The Islamic concept of Hell differs in details from the Christian concept. Both are equally terrifying and are used to control believers. Islam refers to Paradise rather than Heaven, but the belief is similar. In Paradise, good Muslim men enjoy very pleasant sexual rewards. It is unclear what women get there.
Sects
There are two major groups within modern Islam, the Sunni and the Shea. The schism between the Sunni and Shea is akin to the schism between the Roman Catholic Church and the Eastern Orthodox Churches, and occurred for very similar reasons. Protestantism developed partly due to dissatisfaction with the authority of the Pope. Similarly, the Sunni and Shea Muslims disagree whether the male descendents of Mohammed are (or are not) the only people with the right to rule.
As with all the Abrahamic religions there are a vast number of sub-Sects of Islam. Among these is Sufism, an ascetic spiritual movement similar in Philosophy to Buddhism (also less warlike) and Fundamentalist groups such as Salafism and Wahhabism. The Taliban and Al Qaeda are said to be associated with these fundamentalist sects.
In common with their brother religion, Christianity, many Muslims have had no problem in killing coreligionists when they disagree with them. They also kill non-Muslims. Adherents of quite a number of religions have been known to kill people, sometimes including members of the same religion and even family members if their voice in the sky tells them to. Buddhists and Jedi are less inclined to do this.