divX is a codec. you download it as an .exe from the home site. DivX Video Player - DivX Video Codec - DivX Converter | DivX.com
it has been said to have spyware, but from my experience is only has adware, which is not harmful at all.
the codec lets you play movies in the divx format, which is what most ripped DVD's or videos are in.
it also has a converter so you can convert these, and a video player. These are both optional extras that you can choose to skip during the install