Porcelain Veneers are like ‘partial’ crowns.  They’re used when only the ‘front’ surface of the tooth needs to be replaced (when the ‘back’ part is OK).  A thin layer of porcelain (like a false fingernail, but more precise) is bonded very securely to cover the front surface and biting edge of the tooth.  Obviously, they look very nice, but they’re also very easy to clean and make a slightly weak tooth stronger.