Module files (MODs) are a class of file formats used to represent music on a computer. They store several patterns or pages of music data in a form similar to that of a spreadsheet. These patterns contain note numbers, instrument numbers, and controller messages. The number of notes that can be played simultaneously depends on how many tracks there are per pattern.
Module files also give a list of the order in which to play the patterns. However, the biggest advantage of MOD family over standard MIDI files is that MODs include their own audio samples and should sound exactly the same from one player to another barring interpolation methods and any errors in players.
Tracker is the generic term for a class of software music sequencers which, in their purest form, allow the user to arrange sound samples stepwise on a timeline across several monophonic channels. A tracker's interface is primarily numeric; notes are entered via the keyboard, while parameters, effects and so forth are entered in hexadecimal. A complete song consists of several small multi-channel patterns chained together via a master list.
Fast Tracker, specifically Fast Tracker 2 (FT2), is a program that was one of the most widely used trackers in the world. It was created by Fredrik "Mr. H" Huss and Magnus "Vogue" Högdahl, two members of a PC demo group called Triton (now commonly known as Starbreeze Studios) which set about releasing their own tracker after breaking into the scene in 1992 and winning several demo competitions.
Scream Tracker is a versatile multi-track digital sequencer created by members of Finland's Future Crew (FC), legendary in the PC demoscene. It was coded in C language and assembly language. Versions prior to 3.0 created STM files, later ones used S3M.