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!
Colorkey lets you pick one color in a sprite (surface); any pixel of that color will be completely transparent. (If you remember .gif transparency, it's the same idea.)
'alpha' is a measure of opacity - 0 for completely transparent, 255 for completely opaque - and can be applied to an entire sprite (as an alpha plane) or per pixel (slower, but gives much more control).
To make the sprites disappear, I would just set them to non-visible, rather than playing around with alpha values.
Pygame alpha values