For example, a website for a realtor may give information about the realtor, where a web application for the realtor may list current properties and manage the contact information for the realtor themselves. So I would suggest that the question is easier to answer if worded in a different way.
"What is the difference between a web-site which transforms data or information in a significant way, according to the point of view of some specific 'user' or 'customer' and a web-site that does not? In the same way that a brochure may have been created using DTP software, a brochure site may still be managed via some sort of CMS or blogging software.
If, on the other hand, it is mostly static files that link to one another, I would call that a web site.
It does not become a webapp It's like whisky and scotch, all web apps are web site, but not all web sites are web app.
A web application or Rich Internet Applications is a web site that does more than displaying content, it has a business logic. generally a website contains static HTML pages and a web application performs some type of work.
Practically, most websites with quickly changing content will also rely on a sophisticated programmatic (and/or database) backend, but at least in principle they're only defined by their output. Web applications primarily allow the user to perform actions.
The web application on the other hand is essentially a - 100% true; I would consider to be an application because it relies on the input of the users to do a specific task (provide answers to programming questions), not the input of one entity to control it's content. Google Analytics, gmail, and jslint are web applications. A university website likely gives information such as location, tuition rates, programs available, etc; it will likely have web applications that allow teachers to manage grades and course materials, applications for students to register for and withdraw from courses, etc. According to my understanding from your answer, stack overflow is an we application.
For instance, you have an i Phone A web application is a software program which a user accesses over an internal network, or via the internet through a web browser.