A veneer is a thin layer of porcelain (roughly 0.5 -1.0mm in thickness) which is custom made to fit over the front surface of a tooth, like a false fingernail would fit over a nail.
Veneers are used to improve the colour, shape and alignment of your teeth. A precise shade of porcelain can be chosen to give the right colour to improve a single discoloured tooth or to lighten all the front teeth. An advantage of the porcelain veneer to the composite veneer, is that it tends to be more resistant to stain over the years due to its more glassy smooth surface.