Cedega is a commercial spinoff of Wine made by a company called TransGaming designed to allow you to play Windows games using Linux. It uses a mapping of DirectX calls to native OpenGL ones to provide a fast rendering environment. There are a few games that do work correctly, but a larger number which do not.
It was previously known as WineX, until version 4.0 (in June 2004).
There is some controversy about the terms of the Cedega license; the code was forked from Wine when the software was BSD licensed, and to get it you either have to pay, or download a free version of the source from CVS. TransGaming are unhappy about people making the software available.