Teeth stains can be broadly classified into two types: intrinsic and extrinsic. Intrinsic stains are those that develop on the inside of the teeth, in the dentin. These stains are usually yellow or brown in colour and are often the result of certain medications or illnesses. On the other hand, extrinsic stains develop on the outside of the teeth and in the enamel. These stains are usually darker in colour and are often caused by smoking or food and drink.  While both types of stains can be unsightly, they can generally be removed with a professional dental cleaning. However, it is important to note that intrinsic stains may require more aggressive treatment, such as bleaching.