A digital single-lens reflex camera (digital SLR or DSLR) is a digital camera that uses a mechanical mirror system and pentaprism to direct light from the lens to an optical viewfinder on the back of the camera.
The basic operation of a DSLR is as follows: for viewing purposes, the mirror reflects the light coming through the attached lens upwards at a 90 degree angle. It is then reflected twice by the pentaprism, rectifying it for the photographer’s eye. During exposure, the mirror assembly swings upward, the aperture narrows (if set smaller than wide open), and a shutter opens, allowing the lens to project light onto the image sensor. A second shutter then covers the sensor, marking the end of the exposure, and the mirror lowers while the shutter resets. The period that the mirror is flipped up is referred to as "viewfinder blackout". A fast-acting mirror and shutter is preferred so as to not delay an action photo.
All of this happens automatically over a period of milliseconds. Fast cameras do this upwards of 10 times a second.
NOTES: A digital SLR allows you to change lenses to suit the situation and your objective!
DSLRs are often preferred by professional still photographers because they allow an accurate preview of framing close to the moment of exposure, and because DSLRs allow the user to choose from a variety of interchangeable lenses. Most DSLRs also have a function that allows accurate preview of depth of field.
Many professionals also prefer DSLRs for their larger sensors compared to most compact digitals. DSLRs have sensors which are generally closer in size to the traditional film formats that many current professionals started out using. These large sensors allow for similar depths of field and picture angle to film formats.
The term DSLR generally refers to cameras that resemble 35mm format cameras, although some medium format cameras are technically DSLRs.