DSLR, expand to form Digital Single-Lens Reflex cameras are the digital cameras that direct light from the lens to an optical viewfinder on the back of the camera through the implementation of mechanical mirror system and Pentaprism.

DSLR Cameras: Most preferred by Professional Photographers

These cameras are widely preferred by professional still photographers as

They allow accurate preview of framing close to the moment of exposure.
Allow its user to choose from a number of interchangeable lenses.
Allows accurate preview of depth of field.
Larger sensors in contrast to compact digital cameras that allow for similar depths of fields and picture angle to film formats, and even high signal to noise ratio.

Digital Point-and-shoot Vs DSLR Cameras:

The reflex design scheme is what primarily differentiates a DSLR from a digital point-and-shoot camera that exposes the sensor constantly to the light projected by the lens and allows the cameras screen to be used as an electronic viewfinder.

HDSLRs: The High Definition DSLRs that were introduced in 2009 had a video mode in addition to the still photography mode. The video mode allowed its user to record high definition motion video. The Nikon D90 was the first DSLR camera to shoot and capture HD video.

DSLR Timeline:

1991: Kodak was the first to release the commercially available DSLR camera, Kodak DCS-100.
1999: Nikon D1, the first DSLR camera to replace film cameras in the professional photo journalism and sports photography field.
2000 (January): Fujifilms Fine Pix S1 Pro, first DSLR camera to have been marketed to non-professionals.
2004: Konica Minolta Maxxum 7D, first DSLR with in-body image stabilization.
2009: Nikon D90, first DSLR camera with video recording.

Market Share: As of 2008, Canon and Nikon had a whooping share of 41% and 40% respectively whereas Sony and Olympus each had only 6% market share. The duopoly of Canon and Nikon is often referred to as Nikanon or Canikon on online forums.

Popular Brands: Canon, Nikon, Kodak, Pentax, Olympus, Panasonic, Samsung, Konica Minolta, Fujifilm and Sigma.