A DSLR or digital single-lens reflex camera is an advanced type of digital camera that provides high-level image quality, performance levels, and manual control options, typically much better than what you'd receive with a fixed lens camera on a smartphone. This type of camera uses interchangeable lenses, while a fixed lens camera has a lens that is built into the camera body and the photographer cannot swap it out.
Although photographers of almost any experience level can purchase and make use of a DSLR camera, these types of cameras are best for photographers who have some experience with digital photography. Because DSLR cameras can cost anywhere from several hundred dollars to several thousand dollars, they're typically better suited for photographers who have enough experience to take advantage of their high-end features.
DSLR Cameras vs. Mirrorless Cameras
DSLR cameras aren't the only type of interchangeable lens camera though. Another type of interchangeable lens camera, called a mirrorless camera, has a different interior design than a DSLR.
The DSLR camera's interior design contains a mirror that blocks light from traveling through the lens and striking the image sensor. (The image sensor is the light-sensitive chip inside the digital camera that measures the light in the scene, which is the basis of creating a digital photo.) When you press the shutter button on the DSLR, the mirror lifts out of place, allowing the light to travel through the lens to reach the image sensor.
A mirrorless interchangeable lens camera (ILC) doesn't have the mirror mechanism found on the DSLR. Light continuously strikes the image sensor.
Optical Viewfinder Design
This mirror design is leftover from the days of SLR film cameras, where any time the film was struck by light, it would be exposed. The mirror mechanism ensured this would only happen when the photographer pressed the shutter button. With digital cameras using image sensors though, the mirror isn't really needed for this purpose.
The mirror does allow the DSLR to make use of an optical viewfinder, as the mirror redirects the light entering the lens upward and into the viewfinder mechanism, meaning you can see the actual light from the scene that's traveling through the lens. This is why you'll sometimes hear a DSLR"s optical viewfinder referred to as a through the lens (TTL) viewfinder.
A mirrorless camera doesn't use an optical viewfinder, because it doesn't have a mirror mechanism. Instead, if the mirrorless camera includes a viewfinder, it's an electric viewfinder (EVF), meaning it is a tiny display screen, showing the same image that appears on the display screen on the back of the camera. These tiny display screens in the viewfinder all have different levels of resolution (meaning the number of pixels they use in the display), so some photographers don't like some digital viewfinders because they may not have a high resolution, resulting in a viewfinder image that isn't sharp. But you can superimpose some data about the camera's settings on the screen in the digital viewfinder, which is a nice feature.
A digital camera model that looks like a DSLR, but that doesn't offer a TTL viewfinder or interchangeable lenses, is often called a DSLR-style camera. It's a fixed lens camera, but it has a large lens barrel and a large camera body that makes it look like a DSLR, both in body design and in size and weight of the camera.
Such DSLR-style fixed lens cameras tend to have a large telephoto capability, allowing them to shoot photos over a long distance, such as the Nikon Coolpix P900 and its 83X optical zoom lens. Even though these large zoom cameras look like DSLRs, they don't have the high-end image quality or fast performance levels that even the most basic DSLR has.