ATC stands for Artists trading card which are originally given by artists as a sample of there works. They are bits of card that measure 3.5 x 2.5 inches, you decorate one side using a range of techniques and then write on the back a title for it, the date and who its made by and then you swap them with others.
Your atcs must measure 3.5 x 2.5 inches.
The only rule with atcs is they are never sold.
I will warn you now though - atcs are very adictive!