In many of our Arizona rural areas private companies are formed to provide water, particularly in un-incorporated areas. This typically will work out for the user much as it would if there was a local government agency, such as a city, providing the service.
Usually when the area incorporates into, say a city, or is absorbed by a growing city, the local water company assets ( facilities ) are purchased by the city.
We see this quite often in rapidly growing areas.
Or, it may mean as previous answers state, a well or possibly a spring.......when you are really out in the boonies.
If you are purchasing land in outlying areas various govenment agencies require that information about the water supply be disclosed to the would be purchaser. Always ask for a disclosure. It should be forthcoming from the seller. If not, think long and hard before purchasing !!