Stereoscopic photography is a broad topic. Here's a brief introduction and a roadmap to the subject.
Our left and right eyes are separated by small distance, so each eye sees a slightly different image. Through the magic of our visual system, our brain fuses both images into one virtual image. At the same time, the differences in the two images gives us a perception of depth, and enables us to perceive the world in three dimensions. This process is called stereopsis. We can simulate the illusion of a three-dimensional (3-D) scene artificially by using two images with a horizontal offset. This is known as stereoscopy. Stereoscopic photography, or more simply stereo photography, is the technique of capturing two photographs to create the illusion of 3-D.
Her's an excellent introduction on stereoscopy and stereo photography by Pascal Martiné.
Here's a short history of stereoscopy on the 3D Alps website.
The Stereoscopic Society has an excellent section on the different ways to view stereoscopic images.
http://www.stereoscopicsociety.org.uk/WordPress/resources-2/viewing-stereoscopic-images/