I am not a Canon shooter (So Canonites correct me if I am wrong), however they have changed their mount over the years. Their newest mount is the EF mount and lenses are sold as EF or EF-S the difference being the size of your "film" where EF-S lenses are made for the smaller digital sensors found in the 20D 30D while the EF are meant for full frame 35mm film cameras and 5D, 1D series camera's. Their older mounts would not work on a new camera without the aid of an adapter.
Nikon has a similar "DX" lens lineup that is for their digital bodies only, the lenses will mount too a 35mm film camera however you will suffer vignetting around the edges of your images. Nikon however has not changed lens mounts and older lenses are also compatible with new DSLR's