Originally posted by Nobody Special
It didn't really make sense to me, but I thought that during Steve Job's demo of GarageBand he kept saying that it worked with both MIDI and USB keyboards.
MIDI is just a protocol that sends messages between sequencers, instruments, synthesizers, etc, telling them what to do. Type "MIDI protocol" or something into google and a bunch of reading material will show up.
You need a MIDI interface, which will have inputs for the MIDI cables and either sits in a PCI slot or connects with firewire/usb/whatever, if the keyboard doesn't have a USB connector.
To the application, however, it's just all MIDI, regardless of the connector.