The easiest way to define the two is like this...SLR stands for single lens reflex and before the age of digital cameras 35mm film cameras were referred to as SLRs. Now that digital dominates the field, the digital equivalent of an SLR is called the DSLR. Both use a single-lens and a reflex mirror to view the image through a viewfinder. The basic difference - one captures images on film (SLR) while the other captures images through a digital sensor (DSLR).