Nonpoint source pollution refers to the pollution resulting from many different sources, instead of a single source. This makes tracing pollution back to a single source, or effectively implementing regulation, difficult.
Common examples are land runoff, seepage and precipitation. It also includes situations such as runoff from farms draining into a river, or air pollution that comes from cars. Although we do know where this comes from, it is considered to be nonpoint source pollution because of the multiple sources it includes (as opposed to a single location, or a single car).