Stereoviews (also known as stereographs or stereoscopic cards) are among the first form of 3D photography. The pictures are taken with a special stereoscopic camera, which has two lenses, simulating the views received by the left and right eye. Two nearly identical pictures are then developed and printed, and are mounted next to each other, usually on a piece of card stock (although occasionally glass). When looked at through a stereoscope, the image can then be seen in 3D.

An early stereoview
Source: Wikimedia Commons
A brief history of stereoviews

The first stereoscopic viewer (stereoscope) was patented in 1838 by Sir Charles Wheatstone. His device used mirrors to create and project a 3-dimensional image to the person looking into the device. It was rather bulky and ungainly, however, and could only be used to view drawings. In 1844, the art of taking stereoscopic photographs was first demonstrated in Germany, and David Brewster, in Scotland, created the first of the modern stereoscopic viewers.

In 1851, Queen Victoria viewed and praised the stereoscopic views presented at the Great Exhibition. Suddenly, they were a must-have item in Europe, and companies such as the London Stereoscopic Company developed methods for mass-production of images. It took a few more years for the views to catch on in the United States, but they eventually did. Shortly thereafter, Doctor Oliver Wendall Holmes developed the Holmes Stereopticon, a hand viewer, which is still produced in limited numbers today.

Stereoviews were regularly produced until the 1940s, when they were, for the most part, supplanted by film.

(Source: HubPages, The geek girl: http://thegeekgirl.hubpages.com/hub/An-Overview-of-Stereoviews)

See also Making a Positive: Stereoview