What is the difference between a flying boat and a seaplane?

A seaplane usually has canoe sized floats in place of a normal undercarriage on an otherwise traditional looking small aircraft. The fuselage of a flying boat looks more like an actual boat, with the entire bottom part coming into contact with the water, allowing for a much larger plane to land and take off with safety.

The 'Catalina' (PBY-5A) flying boat used by the US Navy during WWII had the advantage of also having a retractable tricycle landing gear, allowing it to operate from either land or water - not something that would be possible with a seaplane.