MikMod

A music tracker (short version tracker) is a type of music sequencer software for creating music. The music is represented as discrete musical notes positioned in several channels at discrete chronological positions on a vertical timeline. The file format used for saving songs is called a module file.

A music tracker's user interface is usually number based. Notes, parameter changes, effects and other commands are entered with the keyboard into a grid of fixed time slots as codes consisting of letters, numbers and hexadecimal digits. Separate patterns have independent timelines; a complete song consists of a master list of repeated patterns.

Later trackers departed from solely using module files, adding other options both to the sound synthesis (hosting generic synthesizers and effects or MIDI output) and to the sequencing (MIDI input and recording), effectively becoming general purpose sequencers with a different user interface.

Music trackers like DefleMask and Famitracker are commonly used to make chiptunes.

The term tracker derives from Ultimate Soundtracker; the first tracker software.Ultimate Soundtracker was written by Karsten Obarski and released in 1987 by EAS Computer Technik for the Commodore Amiga.Ultimate Soundtracker was a commercial product, but soon shareware clones such as NoiseTracker appeared as well. The general concept of step-sequencing samples numerically, as used in trackers, is also found in the Fairlight CMI sampling workstation of the early 1980s. Some early tracker-like programs appeared for the MSX (Yamaha CX5M) and Commodore 64, before 1987, such as Sound Monitor, but these did not feature sample playback, instead playing notes on the computer's internal synthesizer. Later, in Rock Monitor 3 and 4 they implemented sample player, usually with short drum samples loaded in RAM memory.

...
Wikipedia