There are several variables in classifying stocks; in practice, one orders raw stock by a code number.
A piece of film consists of a light-sensitive emulsion applied to a tough, transparent base. Originally cellulose nitrate, which is extraordinarily inflammable, was used. In the mid-thirties, film manufacturers introduced "safety film" with a cellulose triacetate plastic base. All amateur film stocks were safety film, but the use of nitrate persisted for professional releases. Kodak discontinued the manufacture of nitrate base in 1951, and the industry transitioned entirely to safety film in 1951 in the United States and by 1955 internationally. Since the 1980s a growing number of films have used polyester film stock.
Film chemistry may produce either a positive or negative image. Camera films that produce a positive image are known as reversal films. But since negative films are much more commonly used, there are terms based on the steps needed to produce a viewable finished print; one speaks of negatives and positives. Obviously there are color and black and white stocks.
Another critical property of a stock is its film speed, or sensitivity to light. Speed determines the range of lighting conditions under which the film can be shot, and is related to granularity and contrast, which influence the look of the image.
Motion picture film is known to be a highly unstable medium: improperly preserved film can deteriorate in a period of time much faster than many photographs or other visual presentations. Owners of home-made films often find that their film can become brittle and unwatchable in the space of a few years. Decaying film stock gives off an odor similar to that of vinegar, which is why film buffs often refer to such decaying as the "vinegar effect."
Finally, one should mention the distinction between camera stocks and print stocks. It is possible to transfer video images to film stocks that can be developed and printed in the normal manner. Theater performances have been preserved that way for many years--the 1964 New York production of Hamlet with Richard Burton, for example, was shot on video and printed as a film that was released in movie theaters (See also: Kinescope). Digital video equipment has made this approach easier, and certain movies such as Timecode (2000) have been produced that way.