This instructable is designed to help you decide where to look and what to consider when starting a midi project.

USB plug and play devices are great for controlling all kinds of musical programs on computers enabling you to record a wide range of sounds from a single keyboard or midi device. Midi devices range from mixing desks to lighting rigs, to DJ scratch and button pads through to Lazer harps, iPads and the standard keyboard. Midi is the way that each of these controllers can talk to computers or to each other and understand the musical messages that are being sent. 
Remember that Midi is not a sound, it is the instructions for the sound. Making a midi controller is a way to create music via a MIDI interface such as a keyboard, an electronic drumkit, or a computer with midi software (GarageBand, mainstage, logic, ableton live, fruityloops, etc).