Veneers are thin, tooth-coloured shells made of either porcelain or composite, that are bonded to the tooth. They are a more conservative way to improve the aesthetics of a tooth/teeth compared to a crown.
Porcelain veneers normally require two visits to be fitted. They are quite colour stable.
Composite veneers on the other hand are normally done in one visit but are less colour stable compared to porcelain veneers.