Bite wing x-rays are the most common x-rays taken in dentistry.
Your dentist uses them to:
A small packet of x-ray film is placed inside your mouth. When you bite on the tab on the side of the packet, the film becomes properly aligned to get a picture of your upper and lower teeth at the same time.
A lead apron is draped over you to protect the rest of your body while your dentist x-rays your mouth.
Dental x-rays use high-speed film, so the amount of radiation exposure is very low. Though machines vary, bitewing x-rays only add as much radiation as you'd get from about 3 extra weeks of natural background radiation. Even if you've recently had other x-rays taken, bitewing x-rays do not add to that exposure.
Bitewing x-rays are a necessary part of regular dental checkups. They give your dentist vital information that she can't get from any other source.