Allah is the Arabic word for God. It is compounded of "al", the definite article, and "ilah", meaning "a god". Therefore, Allah literally means "the god"--somewhat parallel to the capitalized "God" in English. It is used by Muslims world-wide, as well as Arabic-speaking Christians, Jews, and others.
From an Islamic point of view, Allah is the special name of God and is the most precious name because it is not a descriptive name like other Ninety-nine names of Allah, but the name of God's own presence.
"Allah" may have also been used in pre-Islamic times to refer to the god of the moon and travel, worshiped by nomadic Arabic tribes. This Allah was considered to be the ancestor and leader of the other gods, such as the goddesses al-Lat, al-Uzza and Man'at. According to Professor Carleton S. Coon in his book Southern Arabia, "The god Il or Ilah was originally a phase of the Moon God, but early in Arabian history the name became a general term for god, and it was this name that the Hebrews used prominently in their personal names, such as Emanu-el, Israel, etc." The Hebrew form of this name, El, was used as an Old Testament synonym for Yahweh.
see: the 99 Names of God