Simply put, a web site is more like a brochure and a web app is an interactive application. It’s a fuzzy line at best.
Most of the time it’s pretty clear, but when does a web site become a web app? Discourse is a clearly a web application. Your favorite restaurant probably has a web site. But when does it cross that line? Take SitePoint.com itself, is it a website or web app? It has comments, it has dynamic content, the backend is probably very complex, but it mostly just serves articles which would make it a web site in most people’s eyes.