偷來的http://www.mp3machine.com/news/842/

Constant Bit Rate (CBR) encoding means that you encode a file at a fixed rate, such as 128 Kpbs. For many people this is a common method of encoding MP3s. You can usually tell CBR files because they have consistent file sizes and sound quality. OK, file sizes aren't the kind of thing most of you will look out for. We know that.

Variable Bit Rate (VBR) encoding is a method that ensures high audio quality bit-allocation decisions during encoding. The encoder allocates an appropriate amount of data per second, depending on the complexity of the audio file.