Using other words, CAD (such as free Alibre Design) is to make draft (technical drawing) of part and see how it looks like and how match other elements before manufacturing. Few years ago instead of Pc draftsman used drawing board and stylographs. First CAD (more CAD-like) program which I was using was LORI GRAPH for ZX Spectrum.
CAM is (in most cases) kind of simulator to check if project made with CAD will not destroy machine such as lathe or other CNC.
And it is all you need to know about CAD CAM if you taking into consideration which software to use to make technical drafts/specification of some device. If you trying to build some CNC there is a lot over the net - try "open cnc'" with google search.