What is "film speed"?
Film speed is either (and both) a measured rating of the film when it's manufactured and the "speed rating" at which you choose to expose it.
To start with, it's the number printed on the box and/or the film canister. For most modern (non-digital) films it's also bar coded (sort of) on the canister itself. Automatic developing machines read the rating directly from this bar code and process using this code (more on this later).
Film Speed Rating
The most common over-the-counter-at-your-typical-holiday-resort film speeds are 100 and 200 ISO. This is (of course) the area of measurement most impacted by the advent of the digital camera. The mechanics of aperture and shutter speed are similar in digital and non-digital cameras whereas only the concept of film speed carries across into the digital world.
The general scale (for calculating the light stop is:
2, 4, 8, 16, 32, 64, 128, 256...
So... where does the 100, 200, 400, 800 come from???
Well, the general principle is the same. You double the number and this doubles the light coming in.
The principal behind the rating (the speed) is the same as for aperture and shutter speed. That is, one step in the scale is either an increase of 100% or a decrease of 50% in the amount of light needed to get the shot.
I go into more detail on this concept, and the relationships between the 3 main components, on the Light "Stop" page.
The only important thing to bear in mind is that if you are used to using film in the ranges of 100 200 etc then moving to a film of the type 32 or 64 could be confusing because the transition is not linear. That is, 100 is 50% of 200 but 64 (the next normal value found after 100) is not 50% of 100.... sigh... Yet another of photography's little quirks.
That said, if you're using a light meter then this problem is removed from the light stop calculation and you can concentrate on adjusting aperture and shutter speed for your desired effect.
Choosing a film speed
Given that a change of film midway through a roll is not usual, the important thing is choosing an appropriate film at the outset.
A "fast" film (with a high number) is often used in low light conditions and/or where the subject is moving very fast as the film is more sensitive to light. The trade off with this is that fast film is usually grainier - making the image appear blurred or out of focus.
Despite this, for many years now I have been using and have been very happy with Fuji's ISO800 "Press" film which is regarded as a fast film. This is 3 stops more than ISO100 which was the maximum speed you would have considered 20 years ago - an indication of changes in the chemicals and processes used in that time.
In bulb photgraphy terms this can means sitting in a cold and damp environment for one third of the time! (I'm kind of kidding.)
Pushing a film
To be written later