LAME is able to encode your music using one of its 3 encoding modes: constant
bitrate (CBR), average bitrate (ABR) and variable bitrate (VBR).
This is the default encoding mode, and also the most basic. In this mode, the
bitrate will be the same for the whole file. It means that each part of your
mp3 file will be using the same number of bits. The musical passage beeing a
difficult one to encode or an easy one, the encoder will use the same bitrate,
so the quality of your mp3 is variable. Complex parts will be of a lower quality
than the easiest ones. The main advantage is that the final files size won't
change and can be accurately predicted.
In this mode, you choose the encoder will maintain an average bitrate while
using higher bitrates for the parts of your music that need more bits. The result
will be of higher quality than CBR encoding but the average file size will remain predictible, so this mode is highly recommended
over CBR. This encoding mode is similar to what is referred as vbr in AAC or
Liquid Audio (2 other compression technologies).
In this mode, you choose the desired quality on a sqale from 9 (lowest quality/biggest distortion) to 0 (highest quality/lowest distortion). 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 totally unpredictible.