DivX is an MPEG4 codec which in its original form was an illegal hack of a Microsoft codec in the very late 90′s.
Divx provided the world a high quality codec that had excellent video quality yet small file sizes.
Demand increased so Divx went open source in 2001 and then closed source later that same year as the company turned Divx into a commercial product for profit. Divx versions 5 and 6 are proprietary software owned by Divx Inc.
Divx has lost much of its earlier market share to the Xvid codec Xvid Codec and because of licensing issues is very rare to find new movies and tv shows that you download to be released in this format anymore.