The film speed is usually displayed as a number on the film package and on the film cannister itself. The higher the ISO number, the more light-sensitive the film is. Common film speeds range from ISO 25 to ISO 1600. The finer and closer together these grains are, the better your picture looks and the more you can enlarge it before you notice the grain.
Slow films have a very fine and tight grain structure.
The faster the film or the higher the ISO number, the coarser and more noticeable the grain. So there is a trade-off between speed and grain. You have to consider the lighting conditions and how much quality you need in the photos. If you want to make big enlargements, you should use the slowest and finest grain film that lighting conditions will allow. If you must use a high shutter speed to stop action, or a small aperture to get good depth of field in dim lighting conditions, you will need to use a fast film and put up with the grain.