A DSLR stands for Digital Single Lens Reflex Camera. As the name suggests, these cameras have a single lens which can be removed from the body and replaced with another. The main difference between a point and shoot and SLR camera lies in this very technology.
Point and shoot cameras have 2 lenses. One that the camera sees through, the main lens that zooms in and zooms out, and another that the photographer sees through , the one that you use to preview the image in viewfinder before clicking it. When you click a picture using a point and shoot, the frame that you see in the viewfinder is actually through a different lens than the one that the camera will use to record the image.
SLRs however use the same lens for both of these tasks. Both the photographer and the camera are seeing through the same lens. Take a look at the pictures below for better understanding.
As shown in the image above, DSLRs use this mechanical mirror system to direct light from the lens to the viewfinder. This picture displays the state of the camera when you are looking through the viewfinder before clicking the image. The light coming through lens gets reflected by the mirrors towards the viewfinder, while the shutter remains closed.
This image displays how the camera functions when press the click button. The mirror that was earlier reflecting the light towards viewfinder flips up and the shutter opens so that the light can reach the camera sensor. This entire function happens within a period of milliseconds.
So an obvious advantage that an SLR has is that you see what you get. When you take a picture using a point and shoot camera, the final image is always slightly different than what you saw through the viewfinder because you were looking through a different lens. However in case of a DSLR, because you are looking through the same lens, what you see in the view finder is exactly what your final image will be. However this is is not the only advantage that a DSLR has over point and shoots camera. Some of the other advantages are,
1. Built to last – Made of magnesium-alloy, can stand abuse and shoot in harsh weather conditions.
2. You shoot what you see
3. Better Image Quality due to bigger image sensor
4. Faster shutter speed
6. Better sensitivity to light, can shoot comfortably in all lighting conditions
7. Ability to change and use multiple lenses
8. Control over depth of field
9. Control over shutter speed and aperture
10. Flexibility in terms of controls, shooting options.