In photographic terms, a monopod is rather like a
tripod, but with only one leg. Monopods are extremely useful in situations where the camera needs to be steady, but able to track a moving subject - such as in sports photography. Furthermore, monopods are useful for producing
panoramas, and they are lighter and easier to set up quickly than
tripods.
However, they are inappropriate for producing sharp static images with long exposure times, as you still have to hold the monopod steady with your hand. That said, if you are attacked by an enraged passer-by, a good-quality monopod makes a handy club.