For me a web site is a site that has a main purpose to present information with just a shallow level of user interaction. There might be advanced animations and data retrieved from a database, but foremost purpose is presenting data. A web application is a program, something that would make sense for a user to install on a computer for usage. So the user has a deeper interaction level with the web app than the website, and the primarily purpose is user interaction. A website can contain a web app, and today I believe apps are more and more common which makes me wonder if just stand alone plain web sites will exist in a few years?