This article is part of a series on the various different types of cameras available today.
DSLR stands for Digital Single Lens Reflex. The 'Single Lens' part of the name refers to the fact that the viewfinder looks through the same lens that is used to take the photo (as opposed to a TLR).
The 'Reflex' part of the name meanwhile, refers to the mirror that is used to reflect the light up into the camera's viewfinder. (Actually, the light is reflected up into a pentaprism, which then reflects the light into the viewfinder - the pentaprism is what causes the viewfinder hump on DSLR bodies).
Normally the mirror is in front of the camera's sensor, but when you press the shutter button the mirror flips up out of the way, so that the sensor can be exposed. When this happens, no light can be reflected into the viewfinder, which is known as viewfinder blackout.