Teeth are similar to most things, in they can be eroded by acid.
Acids can come from external or internal sources.
A good example of an internal acid source would be Acid Reflux. Most acid reflux will cause mild to moderate cases of heart burn. The stomach acid will travel up the esophagus toward the mouth. It usually makes it only as high as the level of the heart and this is what causes a sensation that the area around your heart is burning. If the Acid Reflux is more severe it the acid will make it all the way to the mouth. When in the mouth the acids will eat away at the enamel of the teeth.
Another example of internal acid is the condition known as Bulimia. This is where individuals will induce vomiting as a way of losing weight. After eating they want to rid themselves of the food they just ingested before it gets absorbed into the body. As they induce vomiting not only does the food come
up but stomach acids will come up also.
An example of external acids would be sucking on things like lemons or other acidic foods. Many Carbonated beverages are also very high in acid. These acids may not be as strong as acids from the stomach but if the exposure is consistent over a long enough period of time the effects can be just as damaging.
This picture illustrates the effects of acid against your teeth over time.
The tooth should be covered in white. That is the protective layer of the tooth known as enamel. Where the teeth have a more yellow appearance is where the acids have eaten away the enamel. The yellow portion is the layer beneath the enamel which is much softer and much more susceptible to decay. These teeth will need to be crowned quickly to avoid decay from reaching the nerve and the need for root canals.
Acids are very dangerous to your teeth whether from internal or external sources.
Seeing your dentist often will help in diagnosing acidic problems early.