There are three main categories of film sizes: 35mm film, medium format film, and large format film. Within each of those categories are more sizes, but they all relate to the original three.
35mm Roll Film
35mm film is the smallest version, offering the standard size of 24 x 36mm. 35mm film is housed in a canister to keep the film from being exposed to light and is pulled out from the canister when loaded into the film camera to expose individual shots. The sides of the film are perforated so allow the film to wind and rewind, set to the KS-1870 standard. Every frame advances exactly 8 perforations to ensure no frame is exposed more than once and allows for 2mm gaps between frames. 35mm film rolls are typically provided in 24 or 26 exposures. Here is an example of a processed strip of 35mm black and white film.
Medium Format Roll Film
Medium format film is larger than 35mm and is typically seen in 120 or 220 types. Both types are similar except that 120 has 12 exposures while 220 has 24. Medium format film is sized ranging from 6 x 4.5cm to 6 x 6cm (the two most common sizes) up to 6 x 9cm and 6 x 17 cm. Because of the size of this film, it is able to be printed much larger in size without noticeable grain. Unlike 35mm film, the edges are not perforated. Instead, this film uses a leader to wind film from one reel to the next. Here is an example of a medium format negative.
Large Format Sheet Film
Large format film is much larger than both medium format and 35mm format, found in sizes such as 4 x 5in to 8 x 10in. The film is also called sheet film, meaning there is a single sheet of film loaded into a special film holder. The holder is loaded to the back of the camera and the light tight plate is removed to allow the film to be exposed. After exposure the light tight plate is replaced to keep the film safe. Because of its large size, large format photography can be printed very large without much noticeable grain – making this the best film for large prints.