The focal length of a camera lens is the distance between the center of the lens and the film when an in-focus image is formed of an object very far away. The focal length of a camera lens is displayed on the barrel of the lens along with the measurement of the largest aperture and the maker. The focal length of a lens establishes the field of view of the camera. The shorter the focal length is, the larger the field of view. The magnification factor of the picture of an object and the object's actual size can be found by dividing the focal length of the camera lens used by the focal length of a standard lens.
There are three general types of lenses for a camera: normal or standard, wide angle, and telephoto. The focal length of a normal lens for a 35 mm SLR camera is approximately 50 mm. A standard or normal lens produces a picture with a perspective similar to the human eye. The focal length of a wide angle lens is any measurement less than 50 mm, but is typically 28 mm. A wide angle lens makes things appear smaller and distorts the view if the object is too close to the camera. The focal length of a telephoto lens ranges from 60 to 1000 mm. A telephoto lens magnifies the subject while at the same time narrowing the field of vision. These lenses create an image that looks flatter than that produced by a standard lens.
Miriam Janove -- 2002
Source: http://hypertextbook.com/facts/2002/MiriamJanove.shtml