You should google it but here is a basic breakdown.
SLR - Single Lens Reflex
With this kind of camera, you are able to use the viewfinder to get a pretty accurate look at your lens' target.
While on a regular camera the viewfinder has only an approximate view of what's happening on the lens, due to being a completly different device. The SLR, on the other hand, uses a moving mirror to reflect the image coming through the lens towards the viewfinder. When taking a picture, the mirror moves, letting light get to the film.
A DSLR is the same device, using digital inforamtion recording, instead of physical (film).