The biggest difference that I am aware of is that DSLR sensors are smaller than a 35mm film frame. That results in greater focal length. For example a 100mm lens on a 35mm SLR has a focal length of ~140mm on a DSLR (that's for my Nikon D80, but others are similar I am sure).
Other differences can be in the features of lens/camera combinations. Some older SLR lenses might only work in manual AF mode etc. That depends on the manufacturer largely.
I am sure we have some people here that work with a Pentax outfit.
