A wide-angle lens is a lens that has a focal length that is shorter than that of the normal lens. Normal lenses have an angle of view of between 40° and 50°. Wide-angle lenses have an angle of view that is bigger.
24mm (84°), 20mm (94°), 17mm (104°) and 14mm (114°) offer more extreme views (for 35mm film).
This is different for digital photography. Many cameras use smaller sensors. This means that the focal lengths have an angle of view that is different. That way, an 18mm lens mounted on a digital camera of this type gives an angle of view of the 28mm wide-angle lens, namely 75 degrees, for a multiplier of 1.5.
A special kind of wide-angle lens is called fish eye lens.