A crown - sometimes known as a 'cap' - is a dental shell designed to fit right over the top of a tooth, strengthening it.
Your dentist is likely to recommend a crown in cases in which a tooth has become weakened. This can be caused by extensive dental decay; large fillings; accidental damage such as a fracture or root canal treatment. A dentist can also use a crown to hold a bridge or denture firmly in place. A crown holds together the remaining tooth and any dental filling material, making it stronger and greatly improving the appearance of the tooth.
A crown should last for many years. However, the life expectancy of any crown isn't all up to your dentist: it is strongly influenced by factors under your own control, such as your diet and how well you look after your teeth.