Single Lens Reflex (SLR) lenses are meant for use on cameras that use a mirror and prism to allow the photographer to look directly through the lens. These lenses represent the widest selection of choices available, with many manufacturers offering an extremely wide variety of focal lengths, light-gathering capabilities and features. When looking through an SLR, you are actually seeing what the lens is capturing, making framing very accurate.
With the advent of Digital Single Lens Reflex (DSLR) cameras, many manufacturers have introduced additional offerings designed specifically for the smaller crop frame cameras widely available. These crop frame lenses are generally smaller and lighter weight than their full frame counterparts, and often less money as well. Full frame lenses are reverse compatible with crop frame bodies, making them the ultimate in terms of flexibility if moving between sensor sizes.