The earliest practical color photography was the Kodachrome process, which produced color transparencies. Originally used mainly for news reportage, it gradually gained wider popularity. As a medium for serious amateurs, it gained popularity as an alternative to black and white print film starting in about 1945. At this time, color print film had many shortcomings including high cost of film and processing and short print life. Amateurs who could afford slide film and projection equipment used it extensively until about 1970, when color print film began to displace it.
Through about 1995, color transparencies were the only photographic medium used for serious publishing, and were widely used in commerical and advertising photography, reportage, sports, stock, and nature photography. Digital media have since gradually replaced transparencies in many of these applications. The use of slides for artists submitting to juried shows or applying for solo exhibitions, applying to art schools or for residencies (or the like), however, is still nearly universal for a number of reasons, among which is the actual or perceived lack of colour fidelity in digital media.
A slide is a special type of transparency intended to be projected onto a screen using a slide projector. This allows the photograph to be viewed by a room-full of people at the same time. Slides were at one time an important media for presentations, but the widespread availability of LCD projectors has made the market almost go away.
The most common form of modern slide is the 35mm slide, basically a positive-image printing onto the standard 35mm film used in the movie industry, then placed inside a cardboard or plastic shell. Older projectors used a sliding mechanism to manually pull the transparency out of the side of the machine, where it could be replaced by the next image, and it is from this that we get the name "slide". Modern projectors typically use a carousel that holds a large number of slides, and viewed by a mechanism that automatically pulls a single slide out of the carousel and places it in front of the lamp.
Transparency film, in sizes ranging from 35mm roll film up to 8x10" sheet film, are produced by Kodak, Agfa, Konica, Scotch, and Fuji. Essentially all film sold today uses either the E-6 process or the K-22 process, with the overwhelming majority using the E-6 process.
Black and white transparencies can be made with many types of black and white film using modified processing. This is rarely done except in motion picture use.