Followers of Islam profess to believe in the Biblical God known as Jehovah to us. Their term "Allah" is simply the Arabic word for "God" and they profess that to be the God of "Abraham, Moses, David and Jesus". But when it comes to believing in Him as the Bible sets forth, the Muslims are completely off base.
So, you see, in this they completely deny the diety of Jesus the Christ and of the Holy Spirit. Eventhough the Muslims themselves normally say that the Bible is polluted, the Qu'ran states that it is God's word. If you would like to hear something polluted, please read this quote from the Qu'ran's Sura #4 (The Women), verse # 171:
It might as well be the golden calf that Aaron pulled out of the fire. Or the statue of Mary that the Catholics bow down to. Or the bearded old fat guy who comes down chimneys and is deified (and glorified) to children.
Why do I say this? First of all, they do not accept a godhead composed of 3 persons. They contend that God is only one being. They contend that Jesus was not God's son, but His messenger, as was Muhammud. Read these quotes:
"[Faith] in One God is the most important and fundamental principle. It is the bedrock of Islam and the mainspring of its power. All other beliefs, commands and laws of Islam stand firm on this foundation. All of them receive strength from this source. Take it away, and there is nothing left of Islam."
- Abul A'la Mawdudi, Towards Understanding Islam, Islamic Foundation, 1980
"The Islamic interpretation of monotheism is more literal and uncompromising than that of any other religion. Allah is confessed as being one, eternal, unbegotten, unequalled, and beyond partnership of any kind."
- The New Encyclopaedia Britannica, 15th edition, 1992
"Verily, Allah does not forgive the ascribing of divinity to anything beside Him, although He forgives any other sin to whomever He wills: for he who ascribes divinity to anything beside God has indeed contrived an awesome sin."
- [Qur'an (The Women) 4:48]
"People of the Book, go not beyond the bounds in your religion, and say not as to God but the truth. The Messiah, Jesus son of Mary, was only the Messenger of God, and His Word that He committed to Mary, and a Spirit from Him. So believe in God and His Messengers, and say not, 'Three.' Refrain; better is it for you. God is only one God. Glory be to Him - (He is) above having a son." (4:171)
Here are some more links to interesting information about Allah.