35mm has been the dominant film type since Leica standardized the format in the 1920s. 35mm, also known as 135 film, consists of a roll of film held inside a metal/plastic canister. The canister prevents the film from being exposed before or after loading, and is much less prone to failure than the paper backs of other formats.
The image size of 35mm is generally 24x36mm, although some cameras are capable of taking smaller or larger images on the same film. Despite decades of improvement to the emulsions inside, the canister has remained relatively standard for its entire life. Only a few things have been added over time, like lighter materials and electronic codes for ISO.