Actually that is a very simple answer, either it is ground water or surface water.
Ground water originates from precipitation that falls in the form of rain or snow and seeps into the ground, filling the open spaces, or pore space, within layers of sand or gravel (formations) beneath the land surface. Under the ground there is a zone of saturation where the subsurface is completely saturated with water. Layers of sand and gravel in this saturated zone are called aquifers. An aquifer is a geologic formation containing water in quantities sufficient to yield water to a well. The well pumps water to the surface where the Water Treatment Plant treats it to ensure that it is safe to drink. It is then pumped into a storage tank and upon demand by the customer, flows through distribution pipes into the home and ultimately to the faucet.
Surface water also originates from precipitation. The precipitation reaches the land surface and recharges rivers, lakes, wetlands, and other surface water bodies directly. Water is pumped from the water body to a treatment plant and then follows the same path as ground water on its way to the consumer.
More water systems have ground water than surface water as a source, but more people drink from a surface water system. Water utilities treat nearly 34 billion gallons of water every day. The amount and type of treatment (which I will go over in another post) varies with the quality and source of the water. Because surface water systems are exposed to more directly to the atmosphere and runoff they generally require more treatment. Ground water systems have the advantage of being filter as the water seeps into the ground.
Not sure where your water comes from? Well, your water department knows and is required to tell you about it on your Annual Water Quality Report. Go check it out!