DivX Player is the media player by the company that created the DivX codec. Keep in mind there are other programs that are compatible with DivX encoded files. VLC Media Player, Media Player Classic, and KMPlayer are alternatives. DivX Player is criticized for its bundling and its bloatedness.
DivX encoding is used mostly on movie files that are over an hour, because the DivX codec is able to encode the movie to a smaller filesize while retaining high quality video.