A filling is a way to restore a tooth damaged by decay back to its normal function and shape. If you have a tooth that requires a filling, the dentist will first remove the decayed tooth material, clean the affected area, and then fill the cleaned out cavity with a filling material. A filling helps prevent further decay by closing off any cracks or spaces where bacteria can enter.
Dental composite resins are types of synthetic resins which are used in dentistry as restorative material or adhesives.  Synthetic resins evolved as restorative material since they were insoluble, aesthetic, insensitive to dehydration, and easy to manipulate.
The main advantage of a direct dental composite over traditional material such as amalgam is improved aesthetics. Composites can be in a wide range of tooth colors allowing near invisible restoration of teeth. Composite filings can be closely matched to the color of existing teeth. 
If decay or a fracture has damaged a large portion of the tooth, a crown (or "cap") may be recommended. Decay that has reached the nerve may be treated through root canal therapy or through a procedure called pulp capping.