A veneer is a thin layer of porcelain used to cover the fronth of a tooth--usually the front teeth. Veneers can instantly change the color and shape of a tooth and make mildly crowded teeth appear straight. Veneers can require some grinding down of the tooth enamel to ensure a proper fit and appearance.
A crown can also be made of porcelain, and also is used to change the color, shape and "straightness" of your smile. Unlike a veneer, the crown covers the entire tooth.