Around 1983 MIDI (Musical Instrument Digital Interface) was invented as a means of allowing synthesisers to communicate with each other and with sequencers1. It was also meant as a standard to be used across many different makes of synthesiser so you didn't have to use different sequencers for different synthesisers. The adoption of this standard by many makers enabled 'sequencing' to become widespread, as people could easily store and play back music they had created.
Around 1991 a new standard emerged in MIDI, 'General MIDI' in which the basic idea was to have a standard selection of instruments allocated to certain positions in the memory on each synthesiser. Thus a tune created on one synthesiser would sound pretty much the same on any other synthesiser. Though there ended up being three different versions of this from different synthesiser manufacturers, it remained largely the same and cross-compatible. Unfortunately this resulted in synthesisers becoming rather boring, as there was little variation in the sounds and tones produced.
It did, however, provide people with the ability to create backing songs which would work with, and sound (nearly) identical on any synthesiser. Sound 21 'trombone' on a Korg synthesiser would be interpreted and played back as sound 21 'trombone' on a Roland synthesiser, and even though the actual sound might vary slightly, it would be similar enough to suit most purposes.
Due to the fact that you could save whatever you have created, people who were particularly talented started to create MIDI files for commercial distribution. Usually these files were cover versions of popular songs. This was of particular use to people who wished to make a living from performing other people's songs and couldn't be bothered to work out how to play them. Simply order the MIDI file from a company on a disk, load it into your synthesiser and straight away it's all there and ready for you to sing along.
Although they predated the Internet, MIDI files can are available for download from various sources. A handy aspect is that most PC soundcards these days have the built-in ability to play MIDI files. This means that any budding musician has the ability to torture your ears with some badly-programmed MIDI music by setting it as a background sound to their web page.
Searching on the Internet for MIDI files of your favourite band usually comes up with quite a few results. However, be wary of the fact that MIDI file quality can vary - a lot. There are great MIDI versions, and there are horrid MIDI versions.
It is rather unfortunate that many cheaper PC soundcards and 'on board' soundcards on PCs tend to use the most unpleasant and awful sounds for their General MIDI specification. To refer back to the part earlier on where sound 21 was trombone, this would be the same on the PC as on a synthesiser - however, the actual tone is unlikely to sound anywhere near as good.
Another handy use of the MIDI file is saving space in, for example, games. Using your own tune with its own unique sounds is all very well and good, but it might not fit on the CD/cartridge with the rest of the game. Much space can be saved by using a MIDI file (which is small due to just being a set of instructions, rather than a long audio file) to play the standard sounds built into the game console or PC. Quite often on the Internet you can find MIDI files that have been taken from games. This enables fans of the game to enjoy the aural experience of playing the game without actually playing it.
I Want to Make MIDI Files Too!
Feeling creative, eh? Well to create MIDI files you'll either need a synthesiser with built-in sequencer and a disk drive of some sort to save it onto, or a PC MIDI sequencer program. Programs available for doing this include 'Steinberg Cubase' which is the big professional commercial option if you have a lot of money to spare - there are plenty of almost equally advanced shareware and freeware programs that are just a mere Internet search away. But please think carefully before adding the ultimate glockenspiel version of 'Bohemian Rhapsody' to your web page2.