DivX is originally the name of a copy protection for DVD's.
The name however is now used to identify the "hacked" version of Microsofts MPEG4 encoder for video compression.
Using MPEG4 (level 1, 2 of 3) a better compression is achieved in comparison to MPEG2 (used in DVD's). Not only video is differently compressed, DivX uses MP3 or WMA for audio compression as well.

The DivXTM codec is a MPEG-4 based video compression technology that can shrink digital video to sizes small enough to be transported over the Internet, while maintaining high visual quality. To view the compressed version of the video, you need a media player designed to play DivX-encoded content.

I have provided these links for you, below to get and install the DivX codec:

FOR WINDOWS 95/98, NT OR 2000:
(Windows) DivX Codec 3.11alpha - - - - <DOWNLOAD>

FOR MACINTOSH USERS:
(Macintosh) DivX_4.11_Alpha.sit - - - - <DOWNLOAD>
The DivX ;-) codec for the Macintosh platform is in the form of an application that can
decode DivX ;-) movies.

Mac Users: You will need to install Windows Media Player v. 6.3,
and the DivX Player for Macintosh.
The DivX Player requires Microsoft Media Player 6.3
http://a117.ms.a.microsoft.com/f/117/1611/2h/download.microsoft.com/download/
winmediaplayer/mac6.3b2/6.3/MacOS/EN-US/wmp_mac_63beta2.hqx

How to Install Divx:

This tutorial is written using the 3.11 alpha version for windows - other versions may vary.
For the most recent info and versions please visit the DivX homepage.

Install DivX by double clicking the DivX_311alpha.exe.

At the dialog "This setup is not licenced ..." click "OK".

At the "Information" dialog click "Next".

At "Choose destination" dialog click "Next" (if you want you can change the installation location here by either typing the path or by using the "Browse" button).

Now the DivX-Start-menu folder might be in front of your setup, simple minimize (!) it.

Th

e "Finished" dialog appears, select "OK".
Now maximize the Start-menu folder again (or go to the Start-menu and look for Divx in the programs group) and start "Run Me First !!". Answer "OK" on the "do you want to install DivX" question.