I realized that I'm always talking about ACEOs, but I also realized that it must just seem like another Internet acronym to a lot of people! Most of my friends here (*waves*) love ACEOs just as much as I do, so I'll try to demystify them.
ACEO stands for Art Cards, Editions and Originals. Like their sibling the Artist Trading Card, or ATC, they are sized 2.5x3.5" and may be made out of any media. The difference between the two is that ACEOs are sold, while ATCs are traded. Artist Trading Cards came about in the 1990s as part of the mail art movement; I personally discovered them in 2003 when I went to the art college for my first drawing class.
While artists happily trade cards, some people just like to collect them or feel that they might not have any artistic talent to create cards for trade. Personally I love getting cards from people of all artistic ability!
I like to make my art available as prints or originals. If some originals don't sell, I trade them :-) I buy ACEOs too - it's fun to fill up an album or collect a theme, kind of like collecting stamps but more fun.
If you want to learn more about ACEOs or ATCs, there are lots of places on the web! Wikipedia has a nice little list to get started discovering them. And of course, you can also check out the talented ACEO artists of ACEtsy as well!