To get the real power out of the MIDI protocol for composing and reproducing music, you need additional software. A MIDI file is simply a file of all the commands needed to play a certain sequence of music. A PC can store these and then transmit them to MIDI instruments. Alternatively, these can be transmitted to a PC sound card/speaker system, which will play them through MIDI Player software such as vanBasco's Karaoke Player.
A composer could play sounds on a MIDI instrument and have these stored on a PC storage medium. But composers usually make changes during the creative process. Is it necessary to play the revised piece and then store the new commands to make the revised MIDI file? The software that makes composing practical is known as a MIDI Sequencer. You can think of a sequencer as a word processor for MIDI musical commands. The composer can start with a draft MIDI created from playing, and then make changes to the file using the sequencer, even if only correcting an occasional wrong note played. Skillful MIDI composers can even write music directly in MIDI, and play back only to confirm there were no 'typo' mistakes. Using a sequencer, the music editor can easily overdub another part over the first few 'voices' in a composition. A far cry from the pen and ink Beethoven and Mozart had to use!
Today, the PC computer, MIDI instruments and protocol give tools to the musician to compose and play with ease and confidence. A whole new sound world of possibilities, creativity, and enjoyment are within reach of anyone who enjoys playing, composing, and singing - not to forget plain old-fashioned listening!
Use any search engine - you'll find plenty more!