If somebody posts an mpg or mpeg file that they say is DVD Mpeg then you'll need this codec if you'd like to watch it on your computer. This is the MPG2 codec. After installing this all programs like Winamp or Media Player will be able to properly play these videos.

Common file extensions:
.mp2
.mpg2
.mpg

PAL resolution: 720x576
NTSC resolution: 720x480

You can download this codec HERE.