Film speeds range from 100 to 3200 typically. Film speed is often expressed as an ISO setting and many advanced digital cameras allow you to change this setting in a digital way. Film speed really has nothing to do with speed – it would be more appropriate to call it film sensitivity. 100 speed film is “slow” or not very sensitive, it needs a lot of light to make an exposure. 3200 speed film is “fast” or very sensitive.
I’m reluctant to say that slow film speeds are best for bright outdoor situations and fast film speeds are best for action or low light, but that is a general guide line. That said, don’t be afraid to experiment with fast film outside during the day or slow speed films in low light. The important thing to remember is that the more sensitive a film is the more “grainy” your photos will be. Most of the time, 100 speed film will have greater detail and stronger, richer colors than 3200 speed film.
In my opinion, 800 speed is as high as I will go. If I’m shooting black and white, the film can be “pushed” to a higher speed during the developing process. For color, the quality of the image suffers too much at higher speeds. In the photo below, though taken with a digital camera, the quality differs between high and low ISO settings. The effect with film is similar.