Whitening products can bleach the tooth, actually changing the natural tooth color or work by physical or chemical action to help remove surface stains only.
The most common type of professional vital tooth whitening (performed on teeth with live nerves_ uses a gel that is applied directly to the tooth surface. This product contains some form of hydrogen peroxide. Teeth that had root canal treatment could be treated with a whitening agent placed inside the tooth for a few days. There is also a wide array of over-the-counter home whitening kits and home remedies that work sufficiently well in some cases.