Fillings are used to repair a damaged tooth. There are different filling materials available.
The most common type of filling material is an amalgam (silver filling), made from a mixture of metals including mercury, silver, tin, copper and zinc. It usually relies on some supporting dental structure to hold the filling material in place; if a tooth has a very large defect an amalgam filling may not be possible and other restorative options would have to be considered.
A more aesthetic way of restoring teeth is to use a composite (white filling), which is bonded to a tooth and therefore does not rely as much on a retentive cavity design. For this reason it can sometimes be used in cases where an amalgam filling is not possible and otherwise a dental crown would be required.
A dental cement, such as phosphate or glass ionomer cement can be used in cases where a tooth cannot be restored with either an amalgam or a composite filling. This type of filling is usually seen as a temporary type of filling and in most cases the tooth should be restored with either a more permanent type of filling material (e.g. amalgam) or be capped with a crown. Sometimes a cement filling can be used to cover a tooth temporarily to avoid immediate extraction (=tooth removal).