CD tracks are stored as digital data. MP3, and other formats such as WMA, ACC, Ogg Vorbis etc., are compressed forms of the original digital tracks. Such compression schemes are lossy (i.e. you lose some of the information stored in the original form) but the trick is that they generally lose information that doesn't significantly affect sound quality. Where sound quality is noticeably affected then a higher sampling rate or a variable bit rate approach to sampling may improve matters. There are loads of tutorials about this stuff on the web but Howstuffworks.com is often a good place to start.