Here you can learn about arranging and orchestrating with midi
MIDI is a technology that represents music in digital form. Unlike other digital music technologies such as MP3 and CDs, MIDI messages contain individual instructions for playing each individual note of each individual instrument. So with MIDI it is actually possible to change just one note in a song, or to orchestrate and entire song with entirely different instruments. And since each instrument in a MIDI performance is separate from the rest, its easy to «solo» (listen to just one) individual instruments and study them for educational purposes, or to mute individual instruments in a song so that you can play that part yourself.
Most film and TV scores, as well as much popular recorded music is written and performed using electronic keyboards and other MIDI-equipped musical instruments. (Thanks to advances in digital sampling and synthesis technologies, the orchestra playing behind that big-screen block buster is more likely to be the product of MIDI than a real orchestra with dozens of acoustic instruments.)
MIDI also has many other interesting and popular uses. For example, MIDI Show Control is a command and control language used with rides at major theme parks and events at Las Vegas casinos, and MIDI Machine Control is used in recording studios to synchronize and remotely control recording equipment.