Answering this question is indeed one of the chief aims of this site. We quote extensively from a very insightful piece by Abu Iman Abdur-Rahman Robert Squires who writes in an article entitled “Who Is Allah?”:
Some of the biggest misconceptions that many non-Muslims have about Islam have to do with the word "Allah". For various reasons, many people have come to believe that Muslims worship a different God than Christians and Jews. This is totally false, since "Allah" is simply the Arabic word for "God" - and there is only One God. Let there be no doubt - Muslims worship the God of Noah, Abraham, Moses, David and Jesus - peace be upon them all. However, it is certainly true that Jews, Christians and Muslims all have different concepts of Almighty God. For example, Muslims - like Jews - reject the Christian beliefs of the Trinity and the Divine Incarnation. This, however, doesn't mean that each of these three religions worships a different God - because, as we have already said, there is only One True God. Judaism, Christianity and Islam all claim to be "Abrahamic Faiths", and all of them are also classified as "monotheistic". However, Islam teaches that other religions have, in one way or another, distorted and nullified a pure and proper belief in Almighty God by neglecting His true teachings and mixing them with man-made ideas.
First of all, it is important to note that "Allah" is the same word that Arabic-speaking Christians and Jews use for God. If you pick up an Arabic Bible, you will see the word "Allah" being used where "God" is used in English. (Click here to see some examples of the word "Allah" in the Arabic Bible.) This is because "Allah" is the only word in the Arabic language equivalent to the English word "God" with a capital "G". Additionally, the word "Allah" cannot be made plural or given gender (i.e. masculine or feminine), which goes hand-in-hand with the Islamic concept of God. Because of this, and also because the Qur'an, which is the holy scripture of Muslims, was revealed in the Arabic language, some Muslims use the word "Allah" for "God", even when they are speaking other languages. This is not unique to the word "Allah", since many Muslims tend to use Arabic words when discussing Islamic issues, regardless of the language which they speak. This is because the universal teachings of Islam - even though they have been translated in every major language - have been preserved in the Arabic language.
It is interesting to note that the Aramaic word "El", which is the word for God in the language that Jesus spoke, is certainly more similar in sound to the word "Allah" than the English word "God". This also holds true for the various Hebrew words for God, which are "El" and "Elah", and the plural form "Elohim". The reason for these similarities is that Aramaic, Hebrew and Arabic are all Semitic languages with common origins. It should also be noted that in translating the Bible into English, the Hebrew word "El" is translated variously as "God", "God" and "angel"! This imprecise language allows different translators, based on their preconceived notions, to translate the word to fit their own views. The Arabic word "Allah" presents no such difficulty or ambiguity, since it is only used for Almighty God alone. Additionally, in English, the only difference between "God", meaning a false God, and "God", meaning the One True God, is the capital "G". In the Arabic alphabet, since it does not have capital letters, the word for God (i.e. Allah) is formed by adding the equivalent to the English word "the" (Al-) to the Arabic word for "God/God" (ilah). So the Arabic word "Allah" literally it means "The God" - the "Al-" in Arabic basically serving the same function as the capital "G" in English. Due to the above mentioned facts, a more accurate translation of the word "Allah" into English might be "The One -and-Only God" or "The One True God".
More importantly, it should also be noted that the Arabic word "Allah" contains a deep religious message due to its root meaning and origin. This is because it stems from the Arabic verb ta'Allaha (or alaha), which means "to be worshipped". Thus in Arabic, the word "Allah" means "The One who deserves all worship". This, in a nutshell, is the Pure Monotheistic message of Islam. You see, according to Islam, "monotheism" is much more than simply believing in the existence of "only One God" - as seemingly opposed to two, three or more. If one understands the root meaning of the word "Allah", this point should become clear. One should understand that Islam's criticism of the other religions that claim to be "monotheistic" is not because they are "polytheistic" in the classic sense, but because they direct various forms of worship to other than Almighty God. We will discuss the meaning of worship in Islam below, however, before moving on it should be noted that many non-Muslims are unaware of the distinction between simply believing in the existence of only One God and reserving all worship for Him alone. Many Christians are painfully unaware of this point, and thus you often find them asking how Muslims can accuse the followers of Jesus, peace be upon him, of being "polytheists" when they were all "monotheistic Jews". First of all, it should be clarified that the word "polytheist" doesn't really sound right in this context, since to many it implies simply believing in the existence of more than one God. So in an Islamic context, "associators", "man-worshippers" or "creature worshippers" might be more accurate and appropriate terms - especially since Christians believe Jesus to be both "100% God and 100% man", while still paying lip-service to God's "Oneness". However, as we previously touched upon, what is really at the root of this problem is the fact that Christians - as well as the members of other religions - don't really know what "monotheism" means - especially in the Islamic sense. All of the books, articles and papers that I've read which were written by Christians invariably limit "monotheism" to believing in the existence of "One Sovereign and Creator God". Islam, however, teaches much more than this.
Suffice it to say that just because someone claims to be a "monotheistic" Jew, Christian or Muslim, that doesn't keep them from falling into corrupt beliefs and idolatrous practices. Many people, including some Muslims, claim belief in "One God" even though they've fallen into acts of idolatry. Certainly, many Protestants accuse Roman Catholics of idolatrous practices in regards to the saints and the Virgin Mary. Likewise, the Greek Orthodox Church is considered "idolatrous" by many other Christians because in much of their worship they use icons. However, if you ask a Roman Catholic or a Greek Orthodox person if God is "One", they will invariably answer: "Yes!”…
…This brings us to a more important point: It should be clearly understood that what Islam is primarily concerned with is correcting mankind's concept of Almighty God. What we are ultimately going to be held accountable at the end of our life is not whether we prefer the word "Allah" over the word "God", but what our concept of God is. Language is only a side issue. A person can have an incorrect concept of God while using the word "Allah", and likewise a person can have a correct concept of God while using the word "God". This is because both of these words are equally capable of being misused and being improperly defined. As we've already mentioned, using the word "Allah" no more insinuates belief in the Unity of God than the use of the word "God" insinuates belief in the Trinity - or any other theological opinion. Naturally, when God sends a revelation to mankind through a prophet, He is going to send it in a language that the people who receive it can understand and relate to. Almighty God makes this clear in the Qur'an, when He states:
Never did We send a Messenger except (to teach) in the language of his (own) people in order to make (things) clear to them." (Qur'an, Chapter 14 - "Abraham", Verse 4)
As Muslims, we think that it is unfortunate that we have to go into details on such seemingly minor issues, but so many falsehoods have been heaped upon our religion, that we feel that it is our duty to try to break down the barriers of falsehood. This isn't always easy, since there is a lot of anti-Islamic literature in existence which tries to make Islam look like something strange and foreign to Westerners. There are some people out there, who are obviously not on the side of truth, that want to get people to believe that "Allah" is just some Arabian "God", and that Islam is completely "other" - meaning that it has no common roots with the other Abrahamic religions (i.e. Christianity and Judaism). To say that Muslims worship a different "God" because they say "Allah" is just as illogical as saying that French people worship another God because they use the word "Dieu", that Spanish-speaking people worship a different God because they say "Dios" or that the Hebrews worshipped a different God because they sometimes call Him "Yahweh". Certainly, reasoning like this is quite ridiculous! It should also be mentioned, that claiming that any one language uses the only correct word for God is tantamount to denying the universality of God's message to mankind, which was to all nations, tribes and people through various prophets who spoke different languages.”