MIDI stands for Musical Instrument Digital Interface. It is a computer based hardware interface approach developed decades ago for use with various music hardware. Many of the original uses for MIDI were keyboard control, however today a much wider range of MIDI gear is available and in use. This would include:
A MIDI (Musical Instrument Digital Interface) controller is a device that allows the user to communicate with other MIDI devices or MIDI capable software loaded onto a computer. MIDI does this through the use of digital signals known as event messages. These messages can then be used to "talk" to the MIDI software or MIDI device that is connected to the MIDI controller. MIDI controllers do not transfer any audio. Instead, they can tell the connected MIDI device or MIDI software to play back a specific sound, or set of sounds, that might be available inside of the MIDI device or MIDI software. MIDI controllers can also be used to control the specific sound altering parameters available in the connected MIDI device or MIDI software. These parameters may include high eq, mid eq, low eq, sweeping filters, oscillators, delay, effect timing, pitch, and modulation just to name a few.
MIDI options can be used to create and make live music and/or to control music related devices. Keyboards can use MIDI to send note messages to other MIDI sound modules. This way a note played on a single keyboard can produce sound from multiple MIDI sound modules.
MIDI is also used in many environments as a control approach. Guitar players can use a MIDI pedal like the FCB1010 to control preset patches on their MIDI based guitar effects units. Hardware from Behringer (like the V-Amp or V-Amp Pro) are prime examples for this use option.