Well I am not sure where to start but to say I have stumbled upon ATC'S and I am a TOTAL fan.
ATC stands for..
ATC ( artist trading card) is a specific size; 2.5 X 3.5 inches or 3.5 x 2.5 inches . In millimetres that is 64 x 89, its up to you which you use comes out the same in the wash.
These cards started life as a kind of business card for artists but have now become a very popular format for artists particularly for swapping on the internet.
Any medium you like unless the theme for a swap retricts your choice.
They should not exchange for a fee although I know some do.
This is my first ATC.
Its something I look forward to exploring.
Why not give it a go.