This is a difficult question, especially in the English language. Linguistically anyone who uses the term "God" is basically saying the same thing: They are referring to the uncreated Creator of the universe. In this fashion, Muslims, Jews. Christians, Hindus, and all others are referring to the Lord of the universe when they use the term "God."
With regard to Islam, the similarities of Allah and Jehovah are greater for two reasons: (1) Islam embraces monotheism, which means "one God," just like Christianity and Judaism, and (2) Muhammad used many of the people cited in the Bible when he created the Qur'an, such as Noah (Surah 6:84), Jacob (Surah 2:132), and Jesus (Surah 3:45-47).
The similarities end there however. Think of Islam' as a form of "medieval Mormonism." Like Mormonism, Islam is based on the faulty premise that the Bible's description of God and Jesus Christ is incorrect. Like Mormonism, Islam teaches that both Christianity and Judaism are false religions, and that Islam through the Qur'an is the only true faith.
Ultimately, this is not an issue of vocabulary; it is an issue of definition. The Allah of the Qur'an is described and defined in a way that clearly shows Muhammad was not presenting the same God. The God of Islam is remote, and not intimate (Surah 112). Allah does not have a son (Surah 2:116), is not a Trinity (Surah 5:72), and does not love unconditionally (Surah 8:53). In Islam, Jesus is simply a prophet, and emphatically is not the Son of God (Surah 5:72), and the angel Gabriel is the Holy Spirit (Surah 2:193).
Remember, when Muhammad was alive (A.D. 570-632), the Bible was already in its complete canonical form. This is not an issue of ignorance. Muhammad, living just over 700 miles south of Jerusalem, knew the Bible and rejected it. Instead, he created a system that changed the nature and attributes of God, but still used the name.
Indeed, these differences cover the entire theology of Christianity. In Islam, there are "angels" that are created from fire (called jinn, Surah 72), a Tree of Zaqqum in hell (Surah 3:62-67), and virgin servants in heaven (Surah 4:57). In fact, even the creation in Islam is corrupted. In Islam, Satan's fall was due to his unwillingness to bow to Adam (Surah 2:34).
In summation, one can state it thus: Islam rejects the fatherheart of God, the divinity of the Son, and the person of the Holy Spirit. You cannot change the nature of the God of the Bible without changing the "god" you are presenting. It is not the same God.
Extracted from the Apologetics Study Bible.