There are multiple types of tooth whitening solutions available. There are essentially three categories that these fit into including pharmacy available products, Take Home (from dentist) packages and In-dentist clinic applied solutions.
When a whitening product is applied to the tooth, it breaks down into water and oxygen ions. The oxygen ions enter the enamel and attack the long stain molecules to effectively break them down to be short and colourless. These short and colourless molecules are then naturally removed from the tooth by saliva. The intensity of whitening is related to the number of oxygen ions available and the length of time they are on the tooth.