Some of the more popular commercial CAS include Maple and Mathematica. Macsyma and Reduce are older programs that are still available. A new system, MuPad, that resembles Maple in certain ways is currently available free. A list of many CAS is in my web links file.
Commercial implementations tend to be expensive .
One typically refers to features like the GUI, notebooks, graphing, etc. It does mathematics numerically rather than symbolically . That is to say its kernel is numerical . It is based on machine floating point arithmetic and algorithms from numerical analysis. It has the ability to do computations symbolically rather than just numerically.