A crescent is the shape you get if you cut out of a circle another circle of roughly the same size, but offset to one side. You might also describe it as the shape the lit side of a sphere takes on if, in respect to the viewer, the light source is behind and to one side of the sphere (as in a crescent moon).
The word Crescent appears in the name of some places:Islamic faith. However, it should be noted that the crescent was not a symbol used for Islam by Muhammad or any other early Muslim rulers. It originated with the Turks and was first seen on the flag of the Ottoman Empire. Since then it has gone on to grace the flags of Algeria, Comoros, Mauritania, Pakistan, Malaysia, Tunisia, Turkey and other Islamic nations. But it is interesting to notice that very few Arab nations use the symbol.
This is of importance since many Christian fundamentalists such as Jack Chick use this symbolism to make a claim that Allah was in fact a moon-god worshipped by the pre-Islamic Arabs. This argument is used to argue that the god of Islam is different from that of the Jews or Christians.