Of all the bizarre techniques that get used in dentistry, the most misunderstood and obscure one is the inlay and onlay. Many times thought to just be a filling, many times understood as a crown, this aesthetic tooth replacement is a little bit of both, but really is neither. Imagine inlays as basically a piece of a crown, made out of porcelain or composite resin, just like crowns, meant to mend holes that are too big to be effectively filled with just a filling, but that do not warrant the destruction of tooth matter necessary for mounting a crown. The inlay or onlay can be shaped in any matter, replacing missing or worn down cusps and other aspects of the crown that may need to be replaced.
To be more clear here is a comparison to fillings:
Fillings nowadays are made out of artificial resin, a composite gradia that is part gypsum, part artificial tooth material, and partially plastic. Some people still use amalgam fillings, which are metallic, but this is rare. The inlay is a piece of a crown, it is usually made out of procera or porcelain, but can be made out of filling material as well.
The function of a filling is to bridge a gap that has been caused by bacterial damage. It is basically like mending a hole in your enamel, and can only mend a hole in your enamel, as if the decay is any deeper, than a root filling and a root canal treatment will become necessary. The function of an inlay or onlay is to rehabilitate teeth back to a state where they can fulfill their essential function, the chewing and biting of food. Inlays are generally bigger and sturdier than fillings, and usually go in to the tooth matter more deeply, healing not the surface, but acting as a piece of the tooth itself.
A tooth coloured filling has no appearance, if done well, it will look like a piece of your tooth, plain and simple. While there are inlays and onlays that look as if though they were just a part of the tooth, and many people prefer these to anything else, there are several other options available as well. Often times, the inlays or onlays will be made out of precious metals or of strange materials, and many a gold tooth is actually just an onlay of gold that covers most of the buccal surface of the tooth in question.