When most people think of a "filling", they imagine an item made out of some sort of material, either metal or plastic that is placed directly in a hole in a tooth, carved to resemble the original shape of the tooth, and then allowed to harden inside the hole to restore the form and function of the tooth. Of course, it also must relieve the pain associated with the cavity.  In fact, these "direct" restorations, though far and away the most common types due to their lower cost are only one half of the equation.

Another type of restoration, less common due to their much higher cost, are called"indirect" restorations. These "fillings" justify their expense by being more durable (in other words, properly cared for, they should last longer than regular indirect restorations), and also more esthetic (better looking because they are actually built by a laboratory technician on a lab bench without the difficulties imposed by the time constraint and the poor access the dentist faces working in a patient's mouth).   Indirect fillings, made in a dental laboratory, are known as inlays and onlays. 

Indirect fillings used to be more common when gold and ivory were the principal dental materials.  With the advent of porcelain laboratory produced restorations, most dentists today prefer the superior strength and esthetics of "full coverage" of the tooth in the form ofcrowns or veneers rather than simply filling cavities with laboratory processed gold or porcelain fillings.  For a side by side comparison of the characteristics of all indirect filling materials please see my reproduction of the American Dental Association  Table of indirect restorative dental materialsThe types of fillings

There are three major types of direct filling materials (direct fillings are placed in a prepared hole in a tooth, carved or molded to look like a tooth, and then light cured or allowed to harden;

• silver amalgam, made of a mixture of an alloy of silver-tin and liquid mercury
• composite (combination of glass/porcelain particles in a plastic matrix)
• temporary filling materials

There are also three major types of indirect filling material;

• gold (and other semi precious metals)
• fused porcelain
• composite (There is an indirect form of composite which some dentists use.)  

For a side by side comparison of the characteristics of all non temporary direct dental materials, please see my reproduction of the American Dental Association