The line between a website and web application is blurry, but usually a web application is constructed dynamically based on user input.
Anytime a website has a user login system, that’s an indicator that there is a more complex “application” running at the web address it appears on.
Sometimes a web application will talk very closely with the web server, to make sure that when you visit a particular part of the site, certain information related to the user gets sent out to each individual user.
So, if you were to visit a profile page, the web application would know to send back your profile information.