A CD ripper is a program used to create WAV files from a musical cd. It is used in conjunction with an encoder to create MP3's. Some programs perform both functions.

These programs often vary in the quality of audio output as well as&ease of use.

An example of a sub-standard ripper is the ubiquitous AudioGrabber, and also anything Real makes.
An example of a very good ripper/encoder package is NexEncode.