Each "sample" is a small unit of sound: a piano note, a symbol crash, a snare hit, a single guitar pluck, etc. In most tracker music files, most of the space is due to the compression of the raw sample waves; only a little space is used by the actual arrangement of the music notes themselves.
https://www.youtube.com/watch?v=7MSFW8pZ-_4
It has a pretty distinct feel to it, IMHO.
https://www.youtube.com/watch?v=2yDVM77lGlM
Also noteworthy is that you can open these games' soundtracks in a tracker like http://openmpt.org/ to see the arrangements and play samples individually. I tried making music that way a while back and was sad that all my tunes sounded as shitty as the samples I was working with, then opened up the Deus Ex OST and found that his samples were worse, and he was working with fewer channels and filters; it's just pure skill in arrangement that makes the music so great :)
Some other good sites to look at:
http://modarchive.org/ - huge tracker music archive site
https://www.scene.org/ - demoscene information and archive
https://pouet.net/ - demoscene information and archive