it is a CODEC (Compressor Decompressor)
as of now its on the latest version 6.0
and belongs to a family of MPEG4 which is now the reigning new standard for video and audio because of portability to other devices like cellphones and other electronic gadgets
people might be confused that DIVX and MPEG4 is the same. -- they are different
MPEG4 is a standard while DIVX is a CODEC,
Other MPEG4 Codecs
are as follows
XVID, MOV, AAC, DIVX, ASF, RM, WMV, OGM, OGG, Nero Digital Files (MP4) etc
This should be the criteria of buying DVD player that says its an MPEG4 capable player, other claims they are DIVX player so it was tested according to DIVX Codec but not 100% on other MPEG4 CODEC.
Hope it helps