This question may look "stupid" but can you give me an short or longer Introduction how to use MIDI?

I have heard many (good) things about it and about triggering Buttons while a running show, and that you can use a MIDI Board or Instruments etc, but I don´t know what I can use for it, how to configure it, how to set it up and so on!
Thanks
Lighter007
PS: This function may be also useful for new users so it would be nice to have it in the FAQ.
