What is the difference between credits and your cash balance on your account?
The major difference that you can see is that credits will expire within a set amount of days (usually a week) while a your balance will not expire at any point. Credits can be acquired by doing promotions from Gameflip from various sales they promote (like this most recent one where, if you buy something that is $20 not including shipping, you will get $2 in credit back.)
does that clear things up?
So the difference is credits just have a expiration date?
Correct! You either spend it before the expiration date or it disappears.