About Audio Units
Audio Units is a plugin interface for connecting software effects and instruments to sequencers and other audio software. It was developed by Apple Computers and is available exclusively on Mac OSX. AU competes with the VST plugin architecture, which provides similar functionality. Its even possible to use VST plugins within an Audio Unit host using FXPansions VST to AU Adapter. Notable sequencers which support the Audio Unit standard include Apple's flagship Logic Pro and simplified version Logic Express and entry level Garageband. Other sequencers include Ableton Live, and Digital Performer.
AU instruments generate sound in response to MIDI input from a sequencer or keyboard. Instruments fall into two main categories; samplers - which play back recorded sounds, and synthesizers - which generate sound algorithmically.
Apple - creator of the Audio Unit standard
KvR Audio - Audio Unit & VST resource - community site with forums, reviews, annoucements and database of freeware and commercial plugins
Wikipedia (Audio Units) - wiki entry with information on Audio Units
DestroyFX - free creative Aunit Units effects
Apple AU Developer Info - information on creating Audio Units