An example of a population would be all the blue crabs that live in the Chesapeake Bay.
An example of a community would be all the different species that live in the Chesapeake Bay. The community of the bay includes; bay grasses, algae, bacteria, oysters, crabs, many species of fishes, etc.
This video discusses the levels of organization in ecology and includes a discussion of the difference between populations and communities.
Hope this helps!