Islam means "submission". It holds that the true name of God is Allah and that Mohammed is his prophet. True followers of islam seek to follow the sayings of Mohammed as recorded in the Qu'ran. Most muslims also recognise the "Hadeeth" as an authoritative book governing muslim life, but some muslims are against the Hadeeth and deny its authority in matters of life and faith.
There are around 1 billion people who would call themselves muslim, though the percentage of those who seek to follow the Qu'ran literally would be much smaller.
Islam believes in one God. It holds that Jesus Christ was one of a long line of prophets of which Mohammed is the greatest. Muslims do not believe that Jesus is the Son of God, nor that he died to pay for their sins.
Muslims believe that they must say that "there is one God, Allah, and Mohammed is his prophet", that they must pray 5 times a day towards Mecca, that they must make a pilgrimage to Mecca once in their lifetime, that they should give 3% of their income to the poor, and that they should not eat while the sun is up during the month of Ramadan. There are many rules and regulations in Islam concerning many aspects of life.
The Qu'ran is full of warnings about eternal hellfire. The muslim has no assurance that he is good enough to enter paradise and avoid hell. Many muslims are inwardly tormented by their failures to keep the rules, and are unsure of their acceptance with Allah. The only sure promise of salvation to the muslim held out in the Qu'ran is for those who die as martyrs in the cause of islam, as long as they do it courageously. This could help explain why so many young men are willing to blow themselves up while attacking the perceived enemies of muslim people. It is a very logical thing to do if you believe the Qu'ran and are serious about attaining salvation.