Discouloring teeth can be caused by medication, as well as food and drink. The teeth themselves do not change colour but rather have stains. The stains allow less light through the teeth causing them to look darker.
A gel is applied to teeth, usually hydrogen peroxide or carbamide peroxide. A light is shined onto the gel on the teeth and the peroxide become oxygen molecules. These oxygen molecules do inside the teeth and break down the stains. The light is blocked less and the teeth get the "whiter" appearance.
A usual session at the dentist will take about an hour or two for the whole process.
-------
This question is also known as: How does tooth whitening work?
