Patient-friendly definition from the DentistsDirectory.org dental glossary.
A dental crown (also called a cap) is a full-coverage restoration that encases the entire visible portion of a tooth from the gum line upward. The natural tooth is shaped down to create room for the crown, which is then cemented over it permanently. A crown restores the tooth's shape, size, strength, and appearance, and — when properly fitted and cared for — can last 15–25 years or more.
Crowns are recommended when a tooth cannot be reliably restored with a filling alone. Common indications include: a tooth that is severely decayed or fractured with insufficient structure remaining for a filling to hold; a tooth that has been weakened by a large filling covering most of its surface; any tooth after root canal treatment (which leaves it more brittle and prone to fracture); a cracked tooth where the crack extends down the root; covering a dental implant; or for cosmetic improvement when veneers are not appropriate.
The most common crown material today is zirconia — an extremely strong, tooth-coloured ceramic that is biocompatible and suitable for both front and back teeth. Porcelain-fused-to-metal (PFM) crowns have a metal inner layer for strength with a porcelain outer layer for aesthetics; they are still used but have largely been replaced by all-ceramic options. Full gold crowns are the most durable option and are sometimes preferred for back molars in patients who grind their teeth, though aesthetics are the tradeoff. Your dentist will recommend a material based on which tooth is being crowned and your specific clinical needs.