In my understanding, a webapp is essentially a "desktop-like" application for the browser, i.e. something like gmail or an online paint program, something that is interaction driven, will probably use websockets for giving a realtime feeling, etc. A website is the traditional static content, like a blog or a news site. Obviously, these are just relative and its a scale so there cross-over and sometimes its hard to define in exactly which category an app/site falls.