Basically the term SLR (single lens reflex) and DSLR (Digital Lens Reflex) refers to design a camera system using a mirror inside the camera that reflects most light into a prism so that the photographer can see the composition of the photo with a peek in the viewfinder with the accurate fit what is seen by the camera lens.
The main difference is the DSLR using digital image sensors, so that the output is a digital image that is saved to the memory card, whereas if the SLR usually refers to cameras that still use film.
In the mechanism design both are similar.
