Point source would be dumping trash or sewage into the lakes or any other way of directly affecting them.
Non point sourc would be runnof or a contamination in a river that flows into the lake, or in other words anything that indirectly affects the lakes point source pollution is relatively easy to find and terminate, but non-point sources are things like pesticides in a farmers field 50 miles or more away that eventually work their way into the great lakes making them very difficult to stop. The most important water protection law in the us ,the clean water act, has helped significantely to reduce pollution in the great lakes for mor information i would recomend the Environmental Protection Agencies website.