A DSLR, short for digital single-lens reflex, is an interchangeable-lens camera that uses a hinged reflex mirror and an optical viewfinder to let you compose through the same lens that takes the picture. Light entering the lens hits the angled mirror and is bounced up into a prism, which flips it the right way round and sends it to the eyepiece. What you see in the viewfinder is the real optical scene, not an electronic preview.
The defining moment in a DSLR is the exposure itself. When you press the shutter, the mirror swings up out of the light path, the shutter opens, and light reaches the sensor. This is the source of the characteristic mechanical slap and the brief viewfinder blackout. The flipping mirror is also why a DSLR offers mirror lockup for vibration-free long exposures and macro work.
The brightness and quality of the optical view depend on the prism. Higher-end bodies use a glass pentaprism that gives a large, bright finder, while cheaper models use a lighter pentamirror that is dimmer and smaller. DSLRs come in both full-frame and APS-C crop versions, and the crop bodies show only the central portion of the lens image, with a correspondingly tighter finder view.
Because the mirror normally diverts light away from the sensor, traditional DSLRs use a dedicated autofocus module for fast phase-detection autofocus through the viewfinder, and switch to slower sensor-based focusing in live view. The single-lens reflex design solved an old problem elegantly: the photographer sees exactly what the lens sees, with no parallax error and accurate framing across any focal length or accessory.
For two decades DSLRs defined serious photography, building enormous ecosystems of lenses, flashes, and accessories around mounts such as Canon EF and Nikon F. Their strengths include a lag-free optical view that never blacks out except during exposure, long battery life since the finder draws no power, and rugged, proven designs. Their weaknesses are bulk, the noise and vibration of the mirror, and autofocus that historically struggled to cover the edges of the frame.
The industry has now largely shifted to the mirrorless camera, which removes the mirror and shows an electronic preview instead. That change brings smaller bodies, full-time on-sensor autofocus with subject detection, and a live exposure preview, at the cost of the optical viewfinder DSLR users prize. DSLRs remain capable and now offer excellent value on the used market, and for a fuller comparison see our guides to mirrorless versus DSLR and understanding camera types.
For someone choosing a camera today, a DSLR can still be a sound and economical choice, especially used, where excellent bodies and lenses sell for a fraction of their original price. The main questions are whether you value the optical viewfinder and long battery life enough to accept a larger body, and whether the lenses you want are still being made for the mount.