Fillings are done to remove decay, and replace the affected tooth structure. It is called a filling because new material fills the hole that removal of decay left.  There are different materials that are available.

Composite/Tooth Colored fillings: Composites can provide a strong yet esthetic alternative for both front and back teeth. These composite resin fillings are bonded to tooth structure allowing minimal removal of tooth structure making them a very conservative restoration. The material comes in a variety of shades to match your teeth and is therefore virtually undetectable.

Amalgam/Silver fillings: Dental amalgam is a dental filling material used to fill cavities caused by tooth decay. It has been used for more than 150 years in hundreds of millions of patients. Dental amalgam fillings are strong and long-lasting, so they are less likely to break than some other types of fillings and are the least expensive type of filling material.  The downside is that the material does expand and contract over time causing fractures within the tooth.  Also it does not form a bond to natural tooth structure necessitating the removal of more tooth structure to lock it in place.