FIMO clay is basically polymer clay. It's pliable, so you can form shapes with it however you like. There are different kinds of FIMO clay, and some are stiffer than others. The stiffer kind does not smear as easily as the "soft" variety making it more favorable to most artists. FIMO clay has to be baked (usually at 275 degrees F) to harden. Once baked, the clay shape is non-reversible. Clay often comes in different colors and shades; hence another reason for its popularity among artists. Most people use it for making jewelry, photo frames, and home deco.
In the U.S. you can buy FIMO clay from most art stores, or online.