Commonly known as an area code, an NPA is the first three digits of a 10-digit North American phone number. The area code does not include the leading "1".
An area code generally corresponds to a geographic area, such as a part of a large city, a whole small city, or a large rural area.
Area codes can overlap. In areas that have run out of phone numbers and a new area code has been introduced, some cities have chosen to overlay the new area code over the same area as the old one, instead of splitting the area in half. This avoids anyone having to change area codes, but does require that everyone in that region dial all ten digits for each call.
Area codes use the "NXX" format, which means the first digit can not be "0" or "1".