Look at the links on the far left.  See the one 
for "www.cels.org"?  Click it, and you will see 
some great FAQ's about cels.

The short answer, though, is that a douga is like 
a finished sketch.  It is clean, sharp, and looks 
nearly identical to the final cel image.  A genga 
is a much rougher sketch and is often done on 
yellow paper (at least, in my experience).

^_^

JP