Along with Judaism and Christianity, Islam believes in monotheism: that there is just one God. This belief is an essential part of the muslim faith, and the declaration of faith by every muslim, the Shahadah, clearly states that 'There is no god but Allah'.

Islam came to Prophet Muhammad at a time where there was much ignorance and idol worshipping was common, to the extent that the Holy Kabah was used as a shrine where the idol 'Gods' were displayed.

The Quran was revealed to stop people from worshipping multiple gods and remind them that there is just one God, the same God that had spoken to Jesus (Isa) and Moses (Musa) then spoke to Muhammad to reconvey His message.

Inter-Islam - www.inter-islam.org

Wikipedia - www.wikipedia.org