When it boils down to it, the main differentiator of a web application and a web site is that an app has much more interaction and is process-focused rather than content-driven. Users come in to achieve a goal: They provide data to the application, they use the application to enhance that data, and then they expect data to come out. They interact with components of the application and expect them to do something that brings them closer to their goal.
– Christian Heilmann in Event-Driven Web Application Design