Oh that's easy.

A genga is a drawing with many rough lines, like 
rough sketches with stamping of which episode it 
is in, the name of the series and sequence 
number. Normally, it is a really rough sketch. 
Sometimes, it even bears the stamp of the 
animator.

Well, a douga is something very neat and clean. 
Often with only a single line tracing the outline 
of the object. Because this is the sketch that is 
used to make the cel, thus, the sketch is very 
neat and clean. 

It carries information such as the color that has 
to be filled in different areas, the shadings and 
such, and sequence number.

Below picture:
Douga (Left) Genga (Right)