For those of you who've been writing, arranging, and producing songs the old fashioned way, I'd like to explain what midi is and what it can do for you. Midi will not guarantee that you write a #1 hit, but it can certainly improve the efficiency at which you write and allow you to experiment in ways you never could before.
You can see my full definition of midi here recordingreview.com Music Glossary.
You may also want to check out this forum thead about midi.
Essentially, midi allows you to record music in a totally different way that the conventional sound into microphone into preamp into computer method can not do. While most of the recording I do still uses microphones (I didn't waste zillions of dollars on mics for nothing!) there are things that midi can do that microphones can not.