Digital Single-Lens Reflex (DSLR)

This camera is named for the reflexing mirror that allows you to frame the image
through the lens prior to capturing the image. As light passes through the DSLR
camera’s lens, it falls onto a reflexing mirror and then passes through a prism to the
viewfinder. The viewfinder image corresponds to the actual image area. When the
picture is taken, the mirror reflexes, or moves up and out of the way, allowing the open
shutter to expose the digital image sensor, which captures the image. Most features on
a DSLR are adjustable, allowing for greater control over the captured image.

Most DSLR cameras also allow the use of interchangeable lenses,meaning you can swap lenses of different focal lengths on the same camera body.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, as well as their comparatively high signal to noise ratio.

The term DSLR generally refers to cameras that resemble 35 mm format cameras, although some medium format cameras are technically DSLRs.


source : http://en.wikipedia.org/wiki/Digital_single-lens_reflex_camera