Film Speed Rating - ISO / ASA
All film has a speed rating, whether digital or traditional. You may see this number called ASA or ISO (both indicate the film's rated speed). The ISO / ASA rating describes how quickly the film reacts to light.

Film speed uses stops, just like shutter and aperture
For example, going from ISO50 to ISO200 buys you 2 stops more light. Slower films are less sensitive and generally require longer exposures / more light. Faster films react rapidly, and can be used in low light situations.

speed rating sensitivity contrast grain
50 ISO/ASA
low
low
low
100 ISO/ASA
medium
medium
medium
200 ISO/ASA
medium
medium
medium
400 ISO/ASA
high
high
high
800 ISO/ASA
very high
very high
very high
lower numbers = slower films = need more light = longer exposures
higher numbers = faster films = need less light = shorter exposures

*Digital cameras also have a film speed rating, usually around 50ASA. Instead of buying different film for a digital camera, you can adjust the ISO/ASA, just like "real" film.

Film Speed vs. Contrast and Grain
The trade-off between slow and fast films is quality. Slow films generally produce sharper, more detailed images, while faster films often have higher contrast and grain.