Stereoscopy, sometimes called stereoscopic imaging, is a technique used to enable a three-dimensional effect, adding an illusion of depth to a flat image. Stereopsis, commonly (if imprecisely) known as depth perception, is the visual perception of differential distances among objects in one’s line of sight. There are a number of visual cues that help us to see things that way. If one object partially hides another, for example, we understand the one in front to be closer. Objects and patterns grow smaller as they recede and vertical lines converge; objects in the distance are hazier and less deeply colored, with a shift towards the blue end of the spectrum.
3-D TV is a television display technology that ...(WhatIs.com)
The perspective difference between objects seen through the left and right eyes (binocular disparity) and our accommodation through focusing completes stereopsis for normal viewing. 3-D TV (and movies as well) typically work by presenting two separate images – one for the right eye and one for the left – that are incorporated through the use of specialized glasses.
Another technology, known as autostereoscopic imaging (auto 3-D), is screen-based and does not require viewers to wear special glasses. There are two classes of autostereoscopic displays. One type tracks the viewer’s head position to ensure that each eye is presented with a different view. The other type of display uses multiple perspectives of each frame presented simultaneously so that, within a given range, a viewer will see separate perspectives with each eye.
Wikipedia offers a more detailed explanation of stereoscopy.