What are they? Glad you asked! An A.T.C. is an "Artist Trading Card". Typically made in the size of a standard baseball card (2 1/2" x 3 1/2").
A.C.E.O.'s are "Art Card Original or Editions.". The biggest difference is that these are for sale while A.T.C.'s are for trade. A wonderfully economical way to own a piece of original work by your favorite artist!