What is the difference between point-source and nonpoint-source water pollution?
Point sources come from discrete pipes flowing into a river for example. Non- point are more diffuse sources that don't come from one single pipe or outlet.
Point sources are for example, water discharging from an industrial plant of some kind or a waste water treatment plant. Non-point sources include run-off from agricultural lands that may wash fertilizer or other chemicals into lakes or rivers - this may occur over thousands of square kilometres.
From an environmental perspective, it is usually easier to deal with point source emitters than non- point.