If you're looking for game modifications and rom hacks, see Game Mod.
If you're looking for the 1960s subculture, see Scooter-Riding Mod.
If you're coming here via an accidental wick from the abbreviation for the UK's Ministry of Defence, see Brits with Battleships.
MOD (tracker MODule) formats are a hybrid of sorts. They combine note data (like a MIDI file) with digital audio samples (typically in a format similar to WAV). To continue the analogy used on those pages: if a MIDI file is a piece of sheet music, and a WAV file an orchestra, then a MOD file is a piece of sheet music bundled with a specific trumpet for use in playing the piece.
The principle used to generate notes from the samples has been around since at least the early 1960s, where it was used by the BBC Radiophonic Workshop to create the first versions of the Doctor Who theme music. Delia Derbyshire at the BBC recorded individual notes on reel-to-reel tape and generated other notes by changing the playback speed. Faster playback speeds produced higher notes and slower ones produced lower notes. By splicing together segments of tape containing these generated notes and mixing the results together, the first version of the Doctor Who theme was brought to life.
Now, what does that have to do with music on a computer? Well, when a computer plays back music in MOD format, it's essentially doing what Derbyshire did by hand over 25 years earlier. The note data in a MOD file tells the computer how fast and how loud to play back the sample data (this is a simplification, but for an introduction it will suffice).
MOD formats originated in the late 1980s on the Amiga, making use of the rather smart Paula chip in the computers. This sound chip was able to play four streams of audio at once, each with a different sample rate. Unsurprisingly, early Amiga MOD formats were limited to four channels (later software tricks boosting this to eight channels), but after migration to the PC, where more processing power was available, variations on the original format and many new formats with more channels cropped up. The generic name comes from the file extension of one of the first and most popular formats.
MOD formats were fairly popular for use with games in the 1990s for two reasons: they gave a good balance between quality and file size, and they were also consistent. Because pure MIDI formats relied on sound banks stored on discrete sound hardware, despite their ease of storage the quality of playback could vary wildly between any given PC, making it far harder for developers to create a signature soundscape and unified experience for all users; MOD files, while somewhat bigger in drive footprint and still a little dependent on the overall quality of sound a card could produce, provided their own instruments and thus gave a vastly more consistent experience to users (which also meant people could talk about a game's music being good, and thus helped word-of-mouth promotion) and allowed for a lot of inventiveness with the sound, since all instruments were creator-defined and not linked to a static sound library. Several examples of games that utilized MOD format music include Jazz Jackrabbit, Death Rally, Epic Pinball, Terminal Velocity (1995), Star Control II, Unreal, Unreal Tournament, Deus Ex, the Crusader franchise, Seiklus and many titles from PopCap Games such as Bejeweled, Plants vs. Zombies and Bookworm. The Super Nintendo also used, in essence, MOD-format music (see below), which is why some of the soundtracks from the SNES often surpass those of systems released much later and a few RPGs in particular got very clever with it.
The Demoscene has extensively used MOD formats and a number of them have also originated with Demoscene groups, the best-known being S3M (Scream Tracker 3 Module), XM (FastTracker 2 Module* ) and IT (Impulse Tracker Module, not to be confused with the other IT). There are archive sites which have literally thousands of pieces of music in various MOD formats, such as ModArchive, scene.org, Hornet Archive and Amiga Music Preservation.
Unlike standards like General MIDI and MP3, "MOD" refers to an entire family of formats, most of which are not well-documented, having evolved organically from the early SoundTracker format. Thanks to this, it is often difficult to determine whether any given MOD file will be playable with any given player except by experimentation. Many (but not all) of the various subformats are broadly cross-compatible within certain well-known subgroups, but even then, there are (usually) minor incompatibilities which are not always obvious. This is among the reasons that, even where MOD formats remain popular for composing music, the file is nowadays often rendered into a non-tracked format such as MP3 for distribution.
The basic idea of MOD and MIDI existed for video games. During the fourth to the sixth generation of consoles, they produced music similar to MOD in that samples were loaded into the audio chip to be morphed and played back programmatically. A notable exception though is Gameboy Advance, due to having only a DAC, requiring software to mix the audio. Programs do exist to convert some console's formats to MIDI, with the caveat that the instruments will have to be defined manually since video game consoles formats don't map to MIDI directly.
Starting with the later Seventh Generation games, though, pure steamed music in an MP3 or OGG-style format became much more popular, as the available processing could now support it and pre-recorded music of any kind allowed for, well, total control of how the music sounded. In fact, OGG has a parameter that provides a hint to applications where to loop to if looping is needed. By the dawn of the Eighth Generation, pre-recorded streamed music had finally taken over completely and relegated use of "raw" MOD files to history (though trackers, as a utility, remain a vital part of most composers' toolboxes, with just about any serious composer having some familiarity with the techniques Delia Derbyshire pioneered more than half a century ago).
For anyone wanting to get into the MOD scene, a good software to use is the free and open sourced OpenMPT, which is quite versatile and not only supports practically the entire MOD family of formats (it can both load and save them), it also supports features found in more advanced music software such as VST's and ASIO.