CBR = Constant Bitrate
VBR = Variable Bitrate
ABR = Average Bitrate

CBR ( Constant Bitrate ) means that the bitrat selected ( 128kbps, 192kbps, etc. ) is constant and does not deviate. CBR is useful for streaming multimedia content on limited capacity channels since it is the maximum bit rate that matters, not the average, so CBR would be used to take advantage of all of the capacity. CBR would not be the optimal choice for storage as it would not allocate enough data for complex sections ( resulting in degraded quality ) while wasting data on simple sections.

VBR ( Variable Bitrate ) varies the encoding used throughout the song depending on the requirement. For example, if the song is quiet or relatively simple, a lower bitrate might be used. VBR is preferred for storage ( as opposed to streaming ) because it creates a high quality output file with a reduced file size.

ABR ( Average Bitrate ) refers to the average amount of data transfered per second. An MP3 file that has an ABR of 128kbps transfers an average 128000 bits every second.