Depending on where you live in the United States, your water system will be designed according to the climate, with pipes being placed in specific areas to reduce exposure and maximize their longevity.
In northern states, for example, or areas of the country where the temperature is known for falling below freezing during the winter, homes are usually designed with pipes placed within the walls of the structure, protected by insulation from subzero temperatures. Even these houses, however, are not impervious to extremely cold temperatures or other issues, like cracks in the wall or decaying insulation.
Pipes in southern states, however, are significantly more vulnerable, as structures are more likely to be designed with plumbing in areas not protected by insulation, sometimes outside of buildings. On top of that, home or business owners may not be as aware of the fact that freezing pipes can become an issue if the temperatures outside only dip below 32 degrees once or twice a year.
No matter where you live, pipes in attics and crawl spaces are always at risk, as these areas of the home are more prone to being ignored - out of sight, out of mind. In addition, many homeowners who have holes in their walls to let cables in for television and internet or wiring for telephones, fail to properly insulate around these openings, allowing cool air to enter the space.
The material that your pipes are made of plays a small part in whether or not the water within freezes: PVC pipes are more prone to bursting than copper because of their corrosive nature. How well the pipes are insulated is the key, as exposure to temperatures below 20 degrees may result in frozen water.