If you don't know what a MOD is or you need the software for MOD, XM, or ULT playback, you should keep on reading. This page is not intended to be a complete tutorial, but it may answer some of your questions.
If you are a "MOD-expert", don't even bother reading on, because it'll be boring for you...
Collectively called as "MOD-music", these file formats provide a unique way of storing a composition. Just like a MIDI-file, a MOD-file contains the actual sequence of notes to be played together with "effects" that may be acted upon a note (like pitch bend, volume and panning changes, and many more). However, unlike MIDI-files, MOD-files also contain samples of the actual instruments that are used in the composition. Because of this special feature, MOD-music require special software that can play them.
What is the main advantage? A MOD-music will sound exactly the same on each and every computer and soundcard. MIDI playback may vary depending how it is implemented on your particular soundcard (meaning, how the individual instruments sound). But since MOD-music contain the instruments within themselves, there will be no variations.
What is the main disadvantage? Well, obviously, it's the size of the file. Since MOD-files need to contain instrument samples, their size can reach several hundreds of kilobytes - which means it takes more time to download them. In comparison, MIDI music are much smaller and rarely exceed the size of 100 KByte.
Recommended links for further study: