Daredevil's post is accurate. However, I would like to point out something cause some might get the wrong idea.
True - you can't use the previous Canon FD mount lenses on EOS bodies, but any EF mount lens (made since 1987) will work on any Canon EOS body. So you CAN use 'old' lenses with modern EOS bodies.
Apart from the introduction of EF-S lenses, the mount has remained largely unchanged, thanks to Canon's decision to totally revamp their lens mount at the dawn of the AF era.
Nikon had a huge pro user base back then, so they went a different route, retaining as much compatibility as practical and slowly changing the lenses. The result is a more complicated situation. Although old F-mount lenses will fit on modern bodies, there is a moderately complex table detailing which functions work with which lens/body combinations (some combinations have partial functionality). Generally, higher-end bodies work better with old lenses.
The newer Nikon lenses are very much like the Canon EF lenses, with full electronic contacts (no mechanical links) and AF motors built into the lens.