Inlays and onlays are used to repair teeth with damage from decay. Both are a more conservative approach than a crown because less tooth structure is removed in peparing the tooth. An inlay replaces the surface inside of the cusps of the tooth. When the decay is more extensive, coverage is extended with an onlay.
This option is similar in cost as a crown is because inlays and onlays are made at an outside lab and are generally made of either porcelain or gold. Because they are very durable, they can last for years.