A wide-angle lens is one that generally has a focal length of between 24mm and 50mm. Anything with a focal length less than 24mm is referred to as ultra wide angle lens. Note that this designation can change, depending on the sensor size. For example, because of the crop factor on an APS-C sensor cameras, such as an entry level EOS, a wide angle EF lens may well be more akin to a Standard Lens. This is why there are dedicated wide angle lenses for the EF-S mount.
Photographers will use wide angles lenses for any situation where they want to get more of the scene in the shot, for example, with landscapes, architecture, and real estate photography.