GarageBand for Mac OS X is a full recording studio for your Mac. It allows musicians to connect microphones, guitars, basses, and other instruments for a fully analog recording session. It has MIDI playback and recording capabilities as well, allowing anyone with a MIDI capable keyboard to record right along with those instruments.
One of the less-touted features, however, is the Loops section. GarageBand comes with pre-recorded MIDI and sampled audio that fit together in various ways. Without knowing to play a single instrument, you can create amazing sounding music with GarageBand, simply using Loops.