The term DSLR is an acronym which stands for Digital Single-Lens Reflex. So a DSLR Camera is a digital camera that combines the mechanics of a single-lens reflex camera with a Digital Imaging sensor; as opposed to a photographic film.
The primary difference between the DSLR and the non-reflex single-lens digital camera is that the viewfinder of a DSLR presents an image that is almost equal to what’s captured by the camera’s sensor and not being captured by the camera’s image sensor and displayed on a digital screen.
OPERATIONAL DYNAMICS OF DSLR CAMERA’S
The Eyepiece/Viewfinder <8> is situated at the back of the camera. When you look through it, what you see is a reflection of information that has passed through the lens <1>; which means there’s a 100% chance of capturing what you see.
Information in form of light passes through the lens <1> and hits the Reflex Mirror <2> which is positioned at a 45 degrees angle inside the camera structure. This light is then directed vertically to a chamber called the Pentaprism <7>. This light is again re-directed by two mirrors into the viewfinder <8>.
When you take a picture by pressing the shutter button, the Reflex Mirror <2> bends backward, allowing light into the camera. The Shutter <3> remains open for as long as it’s been programmed to. This enables information to get to the Image Sensor <4> which then records the image and then the Shutter <3> closes and reflex Mirror <2> returns to its original position of 45 degrees thereby re-directing light back to the viewfinder <8>.
Once the process ends, the camera then processes information from the Image Sensor (4) and converts it into a pre-defined image format and writes it onto the memory card. This process takes less than a second and in most cases, some DSLR Camera’s can replicate this process of writing images more than 10 times within a second.
This is at best a summary of how your DSLR Camera works.
For an expanded view of how it works, you could follow this link and read more on Wikipedia