In a broad way, speed is inversely related to granularity, which is, literally, the size of the grains of silver halides in the emulsion. A fine-grain stock, such as the ones used for the intermediate stages of copying the original camera negatives, is "slow", meaning that the amount of light used to expose it must be high. Fast films, used for shooting in poor light, produce a grainy image. The image actually consists of a mosaic of developed and undeveloped areas of the emulsion, and each grain of silver halide develops in an all-or-nothing way. If the subject has an edge between light and darkness and that edge falls on a grain, the result will be an area that is all light or all shadow. An accumulation of such areas breaks up the visible contours of the object, the effect known as graininess.
Fast films are also relatively contrasty, for the same reason. That is, an area of the image will consist of bright areas and dark ones with few transitional areas of midtones.
In the early 1980s, there were some radical improvements in film stock. It became possible to shoot color film in very low light and produce a fine-grained image with a good range of midtones. In advertising, music videos, and some drama, it is fashionable to create mismatches of grain, color cast, and so forth between shots. These are deliberate; sometimes they are added in post-production.