In short, usually people are talking about 192kbps encoded MP3's when they say "CD Quality" (the higher the bitrate, the larger the file and usually the better the sound quality). However, what it really means to be "CD Quality" is somewhat more subjective.
Since MP3's use what is called lossy compression, it won't matter what bitrate you choose: you won't ever have a 1:1 copy of the orignal CD track (a 1:1 copy 5 minute song is about 50MB!). When an MP3 is made (or 'encoded'), some special tricks are used to make the music file smaller than the original 1:1 copy. When this done, the information that is lost isn't noticeable to a human listener if the bitrate is high enough. If a MP3 is created using a low bitrate, say 96kbps, you will be able to notice the loss of quality.