Controlling the Film
There are many different kinds of film today. These types come specialized for given photographic settings.
SLR (Single Lens Reflex) cameras have light meters inside that help determine the proper aperture ring and shutter speed, but it will not work correctly unless the ASA setting is matched to the number of the film. This setting must be correct so that the light sensitivity of the film can be accounted for.
ASA of 100 is a lower speed or lower light sensitivity. It is good for outside (bright) settings or for non-moving objects.
ASA of 200 a faster speed. This is for less bright settings or objects that are not still.
ASA of 400 is faster still. This film is to be used for indoor settings or for photographing fast moving objects.
ASA of 800 is even faster. This film would be used to capture low lighted objects in a dark setting. Used at a fast aperture speed it would also be expected to get a good image of an extremely fast moving object.
Other specialized film types are available to professional photographers. Photography has come a VERY long way since Muybridge attempted stop action photography.
Film Advance and Rewind
Film comes in a long thin strip made up of a series of squares ready for exposure. The film advance lever is used to advance each square of film after it has been exposed, that is, after the picture has been taken. Most cameras have safety valves which prohibit double exposure, or premature advancing.
When a roll of film is completely exposed, the rewind crank rewinds the strip of film back into its canister. This canister of exposed film is then treated with chemicals during the development process to produce negatives.