Variable Bitrate

In VBR coding, you choose the desired quality level. Then encoder tries to maintain the given quality in the whole file by choosing the optimal number of bits to spend for each part of your music. The main advantage is that you are able to specify the quality level that you want to reach, but the inconvenient is that the final file size is quite unpredictable.

Most modern encoders are able to encode to VBR. Including (but not limited to) MP3 (specially the LAME encoder), AAC, Ogg Vorbis, Musepack, WMA, etc.

Some codecs limited to CBR include AC3 (in theory it can encode to VBR, but no high quality encoder offers that feature), DTS (same thing as AC3, no publicly available VBR encoder) and Real Audio.

All lossless codecs are, by nature, VBR.