Film speed standards are described by ISO 5800:1987 (they charge a fee for the document). The ISO ratings took the place of ASA ratings in America and DIN ratings in Europe. In practical terms, it's the same number as the ASA ratings.
Basically, a lower ISO number means a film requires more light to create a properly-exposed latent image on film. Some specialized films have deliberately low ISOs to record very bright lights (a welding torch for instance) or light over a very long period of time. Other specialized films only record particular sections of the electromgnetic spectrum (light).
Digital cameras have an ISO rating for sensor light sensitivity as well. It's described by ISO 12232:2006. However, it's based on a previous determination of the quality of the image capture based on digital noise as opposed to actual sensitivity (reaction time) of the sensor.
While some camera makers still don't conform to the minimum, most other camera makers have surpassed the minimum threshold. However, they continue to use the same ISO settings to make it simpler for the public.
In other words, digital images may one day surpass the quality available with 25ISO film, but would still be rated as 200 because there are few legitimate reasons to make sensors slower. Likewise, 3200ISO digital images may surpass the quality of 200ISO film. However, the camera settings are more understandable at 3200ISO rather than something labeled "High Speed 200ISO-D."
In general, consumer films with lower ISOs use a finer (silver halide) grain structure than high-speed films. This translates into clearer-looking images because the grains are smaller.
For most PJs, the goal is to use the lowest possible ISO for the light available. This produces the finest quality image. It also produces images which can withstand enlargement and/or crops without a loss of quality.
To get around this limitation (crops & enlargements), many pro photographers use larger film formats to allow the grain structure of faster films to be less invasive to the total image area. In other words, a larger sheet of film recording the same scene can withstand more cropping and/or enlargement before it starts to fall apart.
The only real "creative" effects capable with a film's ISO are at the grain-structure level. Many of these effects can be enhanced by deliberately underexposing and pushing film (to make the grain larger and more dense to create higher contrast) or pulling film to do the opposite.
Otherwise, PJs can exploit the properties of light with lower speed films and capture long exposures on slower film or combining powerful flash with available light in an unusual way. However, the same could be done by simply adding (matching) neutral density filtersto a lens with any speed film.
Likewise, PJs can use higher ISO films to create more contrast in a scene. However, the faster films come with a larger grain structure and loss of quality. But, some people like it anyway.
On the low end, there are specialized "portrait" and "very low contrast" films, which are primarily used by wedding and portrait photographers
Many pro shooters like to cross process film. This is deliberately processing negative films in positive chemistry (E-6) or positive films in negative chemistry (C-41).
Likewise, color film is relatively intolerant of temperature variations. This can be used to create unusual effects with film by deliberately processing the film in chemistry at an incorrect temperature.
For both of these "alternative processes," a lot of experimentation is required to nail a consistent combination of exposures and chemistry to create the desired results. It's important to only change one variable at a time when developing an alternative film process. Otherwise, PJs are never sure which variable created desired results.
Finally, this isn't suggested unless a PJ is desperate for something new, film is subject to heat and breaks down over time. Deliberately warming or holding film for an unusually long time causes the film color to shift. Both of these are considered "faults" of the film - unless, of course, this is done deliberately and in a controlled manner.
Then, the process can be replicated because a single variable was introduced and recorded. Once the process can be replicated, the PJ can resolve issues to exploit the nature of the color shift.
Another way to play with shutter speed is to use photographic paper instead of film. The sensitivity of photographic paper varies by brand. PJs can test to find the right exposure combination, but somewhere between an ISO rating of five to 10 should be about right.
Jonathon Keats has an interesting project at the Hotel des Arts in San Francisco. Using low-speed paper and a pinhole camera, he's making a 100-year photo of a hotel room. Mind you, someone could go in there with high-power strobe and really mess up the results, but it's a novel project.
We'll save the discussion about types and qualities of film for another day.
Enough for now,