Stereo
Stereo photography allows the interpreter to see the ground in three dimensions by viewing overlapping scenes.
To acquire aerial stereo photography a series of overlapping photos must be taken along a designated flight line. So that stereo coverage of an area will be complete, the amount of overlap for each adjacent pair of photos must be greater than 50%.
The Earth in Stereo
Natural and man-made features can be seen in three dimensions with the aid of a stereoscope and two overlapping scenes. With the addition of depth, aerial photos can provide the interpreter with more information on geologic boundaries, terrain characteristics, relative heights, and regional topography.
Anaglyph
Anaglyph (stereoscopic) images in either black & white or color are viewed, using "mutually cancelling" color filters mounted in paper or plastic glasses. In the past, this was a purely photographic process, but now, computer image processing techniques are used to composite the images. Two discreet views are combined in two layers, in the digital image. These views incorporate parallax of the slightly different viewpoints into a integrated stereo image. The composited stereo image must then be viewed with "anaglyph glasses", which use color filters to create a discreet luminance separation of the views, providing a slightly different perspective to each eye.
Stereo image anaglyphed for red (left eye) and cyan (right eye) filters.
For more on Anaglyph images, see the wikipedia page: http://en.wikipedia.org/wiki/Anaglyph_image.