I am new to using pygame and I was wondering if someone could explain the use of alpha values?  I don't quite understand the difference between that and colorkey.
For my current situation I think I want to use alpha values but am not quite clear how.  
In my game I have two sprites with .png files loaded to each surface.  Upon collision I would like both images to disappear (go completely transparent).
I would really appreciate it if someone could explain the basics of alpha value and how to specifically use them in pygame and if it is possible to use these alpha values to solve my problem.
Thanks!
Although if you want the sprites to fade until they've vanished, gradually reduce the alpha value after they've collided. When alpha reaches 0, use   del sprite   if you don't need the sprites anymore.
Pygame alpha values