The easiest way to begin this answer is to quote the words from the glorious Quran, where Allah introduces himself.

Say, "He is Allah, the one and only. Allah, the Eternal and Absolute. He begets not, nor is He begotten. And there is none like unto Him." (Al Quran 112: 1 to 4).

The word "Allah" means, a Supreme Being, who is "The One and Unique in His Lordship, His Names and Attributes and His Divinity". 
The root word for "Allah" is "Al Ilah”", which means the following:

• The Only One worthy of Worship and whose worship is done with absolute devotion
• The Only One through whom the whole creation rushes with absolute love and satisfaction
• The Only One whose power amazes all of His creation
• The Only One who is beyond our comprehension, but He encompasses everything.

• Allah Has no gender (He is neither male nor female)
• Allah is Always singular - Never plural
• "We" is used only as the "Royal WE" just as in English for royalty
• "Allah" is the same word used by Christian and Jewish Arabs in the Bible

For human comprehension of above characteristics, Allah described himself in the Quran as follows:

Allah - there is no deity except Him, the Ever-Living, the Sustainer of [all] existence. Neither drowsiness overtakes Him nor sleep. To Him belongs whatever is in the heavens and whatever is on the earth. Who is it that can intercede with Him except by His permission? He knows what is [presently] before them and what will be after them, and they encompass not a thing of His knowledge except for what He wills. His Kursi extends over the heavens and the earth, and their preservation tires Him not. And He is the Most High, the Most Great. (Al Quran 2:255)

Moreover, it is a unique name and is much different than the general English word “God”. Muslims prefer using the name “Allah” other than “God” because the name “Allah” cannot be tampered with unlike the English word “God”. The English word “God” takes some prefixes and suffixes which changes the unique existence of “Allah”. For e.g. we generally see people add suffixes, such as father and mother to god to mean God father and God mother. This does not happen with the word “Allah” and its uniqueness is always maintained.