Difference between DivX and Xvid?

First, an example, what do these things have in common?

1. Microsoft in Redmond, WA
2. Apple in Cupertino, CA
3. LAME, the open source project on SourceForge

Answer: all of them offer a MP3 audio encoder.

Likewise...

1. DivX Inc. in San Diego, CA
2. Xvid Solutions in Hof, Germany
3. XviD, the open source project on SourceForge

All of them offer an MPEG4 Advanced Simple Profile (ASP) video encoder. The Xvid Solutions encoder is the same as the XviD open-source encoder and often appears in free software. The DivX encoder is proprietary and only appears in DivX Certified software such as the free DivX Converter.