Even before getting into the SLRs, let me tell you one big difference between a Normal Point and Shoot camera and a SLR camera. Typical Point and Shoot Cameras (excepting Bridge / Prosumer Cameras) have a Viewfinder which is independant from the lens. The Viewfinder would be looking at a frame that is not exactly the same as what the Lens is viewing.
Here, The viewfinder is looking at the Flame of the candle. Actually, what will be clicked is the bottom of the candle, which is not the desire of the photographer. This is called a Parallax error. And this explains why most of the Tall people loose their heads in a group photo.
SLR cameras got a round about way to get this fixed. Tada ! This resulted in what is called a Pentaprism.
Let me paste exactly what was found in Wikipedia for this picture.
A cross-section (or 'side-view') of the optical components of a typical SLR camera shows how the light passes through the lens assembly (1), is reflected by the mirror (2) and is projected on the matte focusing screen (5). Via a condensing lens (6) and internal reflections in the roof pentaprism (7) the image appears in the eyepiece (8).
When an image is taken, the mirror moves upwards from its critical 45
degree angle in the direction of the arrow, the focal plane shutter (3) opens, and the image is projected onto the film or sensor (4) in exactly the same manner as on the focusing screen.This feature distinguishes SLRs from other cameras as, the photographer sees the image composed exactly as it will be captured on the film or sensor
So this is why LCDs in Digital Cameras can't show what exactly is seen in the Optical Viewfinder.
Then came the Electronic Viewfinder (EVF).
To be contd… (On Live Preview)