I'm new to this medium but so far this is what I have discovered.
ATC = Artists Trading Card, these are to be traded and not for sale.
ACEO = Art Card Editions and Originals- These cards are available for swap or sale

Both cards ATC'S and ACEO's are mini pieces of art created out of every medium imaginable. They can be hand drawn, painted mixed medium or collage etc... however as diverse as the medium there is one set rule, each card must not exceed 2.5"x 3.5" inches in size.

One thing I know for sure they are highly additive and extremely fun to create. I have been a collector of paper, vintage photo's and doo dads for years. I bought and bought never really knowing what I was going to do with all this stuff I just knew I had to have it!!! AHHH now I can rationalize all my guilty purchasing I'm gonna use this stuff to create mini pieces of art work. Seems like a win win situation I buy therefore I create, one thing just flows into the other.