A DSLR camera stands for "digital single-lens reflex", this means that there is a sensor inside the camera, when this sensor is exposed to light it will print a digital image what is displayed digitally. The view finder within a DSLR is exact to the lens with use of a mirror, once the button is pressed to take a photo this small mirror moves up and allows a small amount of time for the sensor to pick up the light. This causes the view finder to not show an image for the second that the photo is taken, the speed of the sensor can be altered by changing shutter speed.