ACEO stands for "Art Cards, Editions and Originals". These cards have one main rule - they are 3.5 inches by 2.5 inches - the size of a trading card.
Cards are also sold either as originals or editions. Make sure you know which you are buying! If it is a limited edition print it should be numbered and signed, usually on the back.
These miniature artworks fit in widely available card sleeves and pages, and can be stored in an album or framed for display. ACEOs are very collectible form of art.