The multitrack of a song is basically the studio recorded version (or a cover in the case of GH2) separated into different parts. For example, a Guitar Hero rip may consist of vocals, guitar, bass, and drums, yet properly released files for a song may have up to around 20 different tracks. One of the most popular formats is MOGG, which Audacity (download here) can open.
When using a MOGG file, the left and right tracks get reset to the center, so to stop it distorting, either pan the tracks left and right, or merge the two together to make it stereo
The other files are usually found as MP3s, OGGs or WAVs, and a good program to use to open all of these is Irfanview (download here), or Audacity again if you'd like to play them all at once.
(Both of the above programs are free)
I won't be uploading any of these, but as far as I'm aware, the stems of a song are the loops as opposed to the tracks, so while you are unable to play the entire song, you will be able to remix it pretty well.
As some of the stuff dates back to the early guitar heroes, not everything will be high quality.
