Average/Available Bit Rate encoding primarily for MP3 files, though can conceivably work with anything frame-based. In function it is sort of a middle ground between Variable BitRate(VBR) and Constant BitRate (CBR). It helps maintain broader fidelity but will still give a predictable file size because the sample size will not stretch very far from the core selected bitrate.
ABR is like CBR, but with an unlimited amount of bits to play with. It works by each frame using the smallest possible bitrate which can encode the frame with the desired number of bits.
The VBR Camp generally sees ABR as a neutered version of VBR, and the CBR camp usuallly sees it as pointless excess. My jury is still out on it, but I'm liking it more than VBR but since it's part of the freely-available gpsycho, you can judge for yourself.