The size of a piece of film
, having usually a close relationship with the size of the image
you shoot on it.
In photography you usually talk about small format, medium format and large format. Occasionally miniature format is mentioned, found in niche applications like the Minox spy cameras.
Film format snobs are not unknown, but of course the wise photographer knows that there is not one best format, the issue being largely one of fitness to a particular purpose or shooting style, or availability of hardware for a given format.
Interestingly enough, the film format that currently enjoys the most popularity, namely 135, at the beginning was not considered "serious".
Henri Cartier-Bresson's work changed that perception.
Film format is important also in filmography, but I know squat about that: someone do a writeup on that, pretty please :-)