Sensitivity of Film

Different films have different sensitivities to light.
To compare the sensitivity of films we look at the ISO number.

64 ISO is half as sensitive as 125 ISO.
32 ISO is half as sensitive as 64 ISO.

If we shot a correctly exposed negative using 64 ISO film using f8 at 1/60 s, then we can use different f-stop and ISO film to capture the same scene. (Assume all exposures are made at 1/60 s.)

When the film is developed, a higher ISO produces larger grains of silver on the negative. These larger grains show up in the picture. Fine grain films will be able to show more detail, although both types of film can be used to obtain "sharp" pictures. (The "sharp" look of a print actually depends on the sharpness of the grains of silver.) You can process either "fast" or "slow" films to obtain the result you desire.