Film speed ratings are determined by the film's sensitivity to light. Faster films are designed to gain correct exposure quicker than slower films. The speed rating system is maintained by the International Standards Organization (ISO). Some films, like Kodak's Technical Pan, are not rated so the film's properties need to be examined by the photographer before exposure and developing. Most common film is expressed in multiples of 100 such as 100, 200, 400, 800, 1000, 1600 and 3200, etc. However speeds such as such as 25 and 50 are available.
When the speed rating is doubled, the film's sensitivity to light is also doubled. So a 400 ISO film requires half the exposure time as 200 ISO, but twice as much as an 800 ISO film. Faster films allow more options than slower films, as they are ideal for low light situations where a flash may be impractical to use. High speed is also made for taking action shots, where a fast shutter speed is needed to freeze the action. However, fast film tends to look grainy when it's enlarged. Slower films do have some benefits as they have a finer grain and are designed for taking photos of with greater detail. They are also good for indoor shots with a flash or for outdoor photography in bright conditions.
A rated film can be pushed to behave like a higher ISO film. Which means a 400 film can be pushed to 800. It will be exposed for a shorter period of time than normal. To do this, the film must be developed longer than normal. Likewise, a film can be pulled to act like a slower film. Film can usually be bought for just a few dollars.