MIDI was developed in the 1980s as a solution to certain recording and performance problems. Before MIDI, a musician couldn't switch from one specific set of tones and sounds to another on the same synthesizer without taking valuable time. He would have to use multiple different machines--keyboards, drums, organs, pianos, or whatever he required to get the different sounds he needed. MIDI allowed them all to be connected and controlled through a single keyboard, giving the musician unprecedented ability to develop the sound he wanted.
MIDI works through a device known as a sequencer, which records data representative of the sound to be reproduced. In and of itself, a sequencer can't make any sounds. It simply stores the electronic signals which another instrument (such as a synthesizer or a drum set) can interpret and play back. Experts have likened it to a cylinder on a music box, or the roll of paper in a player piano. Once the data is in place, it remains there for good, and can be reproduced or manipulated as the musician sees fit.
MIDI requires three components in order to work correctly. The first is an input device, such as a keyboard with MIDI capacity, where the sound can be generated and recorded into the sequencer. The second is the sequencer itself, which houses the MIDI file and makes it available to the musician. The third is the sound generator or synthesizer, which can interpret the file and play it back as sound. In the early stages of the technology, the synthesizer was usually a bulky piece of hardware. Today, it can be a simple piece of software which can be played on any laptop.
The primary benefit of MIDI is the ability to manipulate the data to achieve a specific result. Without MIDI, a musician would have to re-record a given sound and proceed from there. MIDI makes for a much more convenient tool: able to adjust pitch, tone, tempo and other aspects of the sound without any unwanted side effects. And because there is no actual audio involved (as in an mp3 file), MIDI files are extremely small--about 10 kilobytes per minute of recorded data--which makes storage much easier than with sound files.
In the 1980s, MIDI required bulky equipment and ate up a lot of data storage space. That was because computers lacked the memory innovations that they do today. With the advance of the computer revolution, MIDI files have become increasingly commonplace. Today, MIDI files are used on websites, video games and numerous small electronics. Karaoke machines absolutely depend upon them because they can adjust the pitch for different voices, and you likely have an MIDI file on your cell phone's sound card, which lets you choose different songs and tones to act as your ringer.