They are small works of are typically (I think) 5x7.5 cm that are collectable.
There are several art card projects going at the moment see the project index. Here is the link to one of them http://www.wetcanvas.com/Community/P...s&proj_id=1124
There are also several being traded on e-bay, so if you do a search you will be able to see what they are.
I also thought there was a forum here but I cannot find it.
Just found it. It is under the minature art forum herehttp://www.wetcanvas.com/forums/forumdisplay.php?f=339