Composite fillings are a tooth-colored plastic and glass mixture used to restore decayed teeth and also for cosmetic improvements of the smile. Composites bond to the tooth to support the remaining tooth structure, which helps prevent breakage and insulates the tooth from excessive temperature changes.

Source: Academy of General Denistry
http://www.knowyourteeth.com/infobites/abc/article/?abc=w&iid=286&aid=1241