I was looking for someone to mention the ISO number (just the most recent numbering system, which also included the ASA and DIN systems). Film that was readily available to the public ranged from a fine-grain ISO of 25 (like Kodachrome 25) or 50 (like Fuji Velvia) to films that had an ISO of 400 or 800.

Quoting Wikipedia: "Velvia has the highest resolving power of any slide film. A 35 mm Velvia slide can resolve up to 160 lines per mm."

You could capture more detail with a lower ISO film, but you also needed more light. For closeup photos you might use an ISO 25 film, steady your camera on a tripod, and use a shutter release cord that kept your hands off the camera. For fast action like sports, where you also need to be handheld, you'd use a high ISO film.

Ansel Adams nature photography? Low ISO.

Shots of pole vaulters, race cars, or skiers? High ISO.

Cameras also let you set the ISO value to something higher or lower than the actual value. This had an effect similar to overexposure or underexposure. For example, there might be times when you needed your 400 ISO film to act as if it were a 1600 ISO film that needed much less light. Doing this usually resulting in some degradation to the image, though.

Lens quality, the ability of the photographer to focus, and other thing mentioned in this thread also play a role.