ACEO stands for Art Cards, Editions and Originals.
More specifically, they are an eBay offshoot of another acronym ATC (Artist Trading Cards). A group on eBay's seller forums started to produce ATC's for sale. Unfortunately, they were not sticking to the original concept (trading). They devised a new name that reflected what these cards were - original works of art and very limited editions that are meant for sale.
There is an official size to ACEO - 2.5" x 3.5" anything more or less cannot be called an ACEO. Most tend to be paper based or thin cardstock, but there are ACEO made out of fabric, glass and wood. (they do conform to the length and width measurements.)
When you purchase an ACEO, most artists will put it in a protective sleeve so that you can handle it easily. Most common are penny sleeves (soft cellophane) or top loaders (a more rigid plastic).
The wonderful thing about ACEO are the potential that they pose for collectors. Almost anyone can start a collection of these tiny works. They are affordable, they are inexpensive to ship and there is a huge selection available to choose your style, subject, or medium of choice.
Okay, so I've started collecting these little paintings, now what do I do with them? Well, firstly, keep them safe, they will increase in value if they are kept in great condition (just like other collector cards or original pieces of art). Keep them dry, out of direct sunlight (keeps the pigment vibrant) and avoid touching the surfaces directly (the natural oils on your hands can damage it).
Popular ways to display ACEO are in tiny frames - solo or in a group. A larger frame with a plain matte or a special ACEO matte which allows you to display a grouping all in one frame.
Another great conversation starter would be to invest in a beautiful coffee table book where you could insert plastic baseball card sleeves. Guests could then flip through your collection and you could store a large quantity of original art in a relatively small space. I've even heard of people who keep a small collection in a fold out style case so that they can take it with them, wherever they go. The possibilities are endless.
If you find these tiny treasures intriguing, I urge you to go to ebay or other artist's websites and type ACEO into the search bar. Thousands of works will come up. Many artist's careers are based on ACEO and their work is increasing in value dramatically.
Become a collector - they are so interesting, unique and fun.