"The general distinction between an interactive web site of any kind and a 'web application' is unclear. Web sites most likely to be referred to as 'web applications' are those which have similar functionality to a desktop software application, or to a mobile app."

https://en.wikipedia.org/wiki/Web_application

An easy distinction I use is: can the user alter the content? If so, it's an application. If not, it's a website.