Texture mapping with RGBA

Hello , when we define a RGBA texture ,we always define R and G and B as a value of 0-255, then what is the A value ?is it must be 0-255 or just 0-1? and what is the difference?
thank you very much!!

Yeah~~

A is the alpha value

One of study case from Nehe shows how to use it

http://nehe.gamedev.net