A digital single-lens reflex camera (digital SLR or DSLR) is a digital camera that uses an automatic mirror system and pentaprism or pentamirror to direct light from the lens through the viewfinder eyepiece.
The basic operation of a DSLR is as follows: for viewing purposes, the mirror reflects the light coming through the attached lens upwards at an approximately 90 degree angle. It is then reflected by the pentaprism to the photographer's eye. During exposure (when the photograph is taken), the mirror swings upward, and a shutter opens, allowing the lens to project light onto the image sensor.
OK, that is all you realy need to know about SLR cams, if you want more techy stuff you can do a google search for the history, how they are made, etc... but for now lets move on to exposure