MIDI: What is it? How does it work?

Midi is just controller data for (musical) devices, that's right.

I don't know what setup you have or want to use, so I'm assuming you only have your PC.

Basically you need a device that creates midi data and another that can creates sound with the data.
And you need a virtual connection to transfer the data from one device to the other.

When you don't have a midi keyboard you can use a program like Bome's Mouse Keyboard http://www.bome.com/midi/keyboard/ or Sweet Little Piano www.ronimusic.com

With those apps you can turn your computer keyboard into a midi keyboard.

You can trigger the wavetable sounds of your soundcard with it or use Hubi's Loopback Device http://members.magnet.at/hubwin/midi.html or Midiyoke http://www.midiox.com/myoke.htm to connect it to a Midisynth.

You select a virtual channel in the keyboard app and then select the same channel as input in the softsynth.

There are tons of free and commercial synths available on the net.

The Buzz is a great modular synth - for free

www.buzzmachines.com

This is a really deep subject, so if you need more info, you ask.
It's also helpful to know what you want to do/achieve.

 

MIDI= Musical Instrument Digital Interface
It's a way for digital instruments (drum machines, synthesizers, sequencers, recorders, etc.) to communicate with each other. The most comon use is for linking synthesizers or modules together so many sounds can be controlled from one keyboard. I use it to "sync" my digital recorder and my sequencer. That way I don't have to use tracks for the drum machine and synthesizers and I can easily go back and edit those parts. The best part is you can start with a very simple drum track and then after the other tracks are laid you can add fills and other changes.

 

Not only musical devices. MIDI can be used to trigger lights, machines and can be integrated as controllers for software (e.g. you can remote control virtual software mixers with a hardware console and a MIDI connection... well, that's a musical device after all, but you catch my drift ).

The world has realised its usefulness for other purposes than just triggering sounds. I guess you knew that already, but maybe not the topic starter.

 

Yeah, that's why I used the brackets, but I should have made it clearer.

I even saw a Midi-controlled coffee machine somewhere...

Wait, know I know what I want for Xmas

 

And it is also used in preamps! I have an ADA MB-1 and you can use a MIDI footcontroller to send program changes that change the program from say, heavy distortion (on program 50), to a low mid's boosted program on "patch" 56. Get it?