alpha maps, I'm assuming you're referring to 3d. Alpha's are basically transparency's in the form of a greyscale image. pure black is usually "not see through" and white is 100% see through and everything grey in between is well, different levels of transparency's. (if this is wrong ...then do the exact opposite
)
in 3d people use these maps in the "transparency" channel of the shader to mimic things like windows, glasses and water. or pretty much anything else you want to be see through, or completely hide.
in video games, or "low poly" work, alpha maps are used to help create the illusion of complex modeling. For instance in a video game, you would not model every wire in a chain link fence. Instead you would have a single polygonal plane (or a few more depending on it's shape) then you would apply a transparency in the "silhouette" of the chain link fence. (does this make any sense? if not lemme know and I'll explain better.)
Anyways, I hope this helps you out.
-agentxi-