In-lays, On-lays

An inlay refers to a filling that usually consists of gold, porcelain, or resin that is used to treat a cavity. An inlay is larger than a filling while being smaller than a crown. The difference between an inlay and a filling is that an inlay lies within the cusps on the chewing portion of the tooth. An onlay is the same as an inlay, except that an onlay is specifically targeted towards replacing a tooth cusp. 

They are major single-tooth restoration options used for filling larger areas of decay where filling inlay would not work. The Onlay covers the cusp of the tooth while inlay only fills the area between the cusps. On the other hand, the crown will cover the entire biting surface of the tooth and the tooth structure above the gum line. The cost of both is almost similar but Onlay can be a little cheaper in price.