A module, or "mod" for short, is a self contained file of modular music. It contains all the samples required for the song, as well as the instructions to play them. Modules come in many formats, such as XM, S3M, MOD, 669, and so on. Each format is generally created by a particular module authoring program; for instance S3M files are created by Scream Tracker 3. There are also a wide variety of players that can play various subsets of module formats.
Modules were extremely popular in the days of the Amiga computer, which was one of the first systems capable of creating and playing them. They were also prevelant in the Demo Scene, where cutting edge graphics and music were combined to create a multimedia experience long before the term was coined.