I should also point out that there are different qualities between encoders. Some are better than others. The mp3 encoding quality of one player will be different to the next. Thus, you can encode the same file from a cd to mp3 with 2 different programs & notice a small difference in sound & perhaps size also. Generally the newer the encoder the better quality. The reknown best mp3 encoder is LAME.

There is a LAME encoder you can get for iTunes.