If you want to get technical, a web app is also a website. The practical difference lies in their interactivity: Websites usually contain relatively static, scrollable content, while web apps are more complex, interactive tools or systems that also happen to be accessed through a web browser. Think (website) wikipedia.org (scrollable, static content) vs. (web application) Shopify (highly interactive, dynamic, has several layers of security, and it needs to do a WHOLE lot of things).