Water has a unique property in which it expands when it freezes. As you can imagine if water freezes in your pipes within your home, the pipes will expand with the water putting them in danger of bursting.
Pipes freeze when they are exposed to extreme cold for an extended amount of time, for example hose bibs, swimming pool supply lines and water sprinkler lines. Also, supply lines in unheated areas run risk of freezing, for example pipes in the attic, basement or garage. With that, pipes that live in the exterior of walls run risk of freezing as there is little to no insulation in those areas.