The purpose of MIDI (Musical Instrument Digital Interface) is to transmit information precisely describing the actions of a performer on his/her musical keyboard.

It is a description of the physical performance on a musical keyboard. The MIDI language contains instructions like : "Press on the 5th white key of the keyboard with a force of 75%, Release this key 2.5 seconds later, Wait a 0.4 second then press on the 7th key, press on the hold pedal, etc...". 

It is a descriptive language which contains a vocabulary of elementary actions that the performer can execute, like pressing and releasing a key of the keyboard, a pedal, handling a button to change the sound, to increase volume...

A MIDI file (.MID extension on PC) does not contain sound but only instructions to create a piece of music. 

It is the construction plan of the house but not the house itself. To hear the sound result, a sound synthesizer is needed either, external to the computer (MIDI connected piano or synthesizer) or internal (present on the sound card or as a software synthesizer).

A synthesizer transforms the MIDI instructions into audible sounds via loudspeakers.

Learn more about MIDI and music software in the Pizzicato manual!