Someone upthread asked why it is important to Muslims. Before Islam came to what is now the 'Islamic World', it was common amongst the tribes of these regions to practice circumcision and FGM in some places, too. So it does have cultural rather than religious origins.
In terms of the religion itself, the belief is that Mohammed was born without a foreskin, so it is to match with that. There's also a hygiene element, the pre-prayer washing ritual is pretty important. There are schools of thought that say that as Mohammed wasn't circumcised but apparently born that way, men don't need to alter their own bodies in any way either.
Also, circumcision is not essential in Islam, but it is recommended. My husband is Muslim, but my sons aren't circumcised. Caused issues with my in laws, but I didn't care. I'm very much against it, and would like to see it outlawed.