A cavity is basically a hole in your tooth caused by decay. To remedy this, we remove the decay and then fill the hole in your tooth. That's why they're called fillings! Dental fillings are generally indicated to help restore cavities that have not reached the nerve of your tooth or cavities that do not encompass the entire tooth. Fillings can also be used to restore chipped or fractured teeth.
Fillings can be done with many different materials, but we generally will use a composite resin material to fill the majority of teeth. A filling procedure generally requires local anesthetic to numb the tooth while we remove the decay. Once the decay is removed, the preparation is shaped to best help retain the filling/restoration. We then place the composite resin and use a special light to cure the material. We then check your bite to make sure everything looks right and polish the filling so you can show it off to your friends.
We will occasionally also use amalgam fillings in certain situations. The benefits of amalgam are generally a reduced cost when compared to composite resin fillings as well as, in general, a better longevity in posterior teeth. Patients will sometimes have concerns about mercury in the amalgam. Here is a link to the American Dental Association's statement on amalgam stating that amalgam continues to be a safe and valuable alternative as a dental filling material. If someone you know has said they've done their research and they disagree with the ADA statement, I highly recommend them to publish their findings in a peer reviewed journal.