What is a web app?
According to Wikipedia:
In software engineering, a web application or webapp is an application that is accessed via web browser over a network such as the Internet or an intranet. It is also a computer software application that is coded in a browser-supported language (such as HTML, JavaScript, Java, etc.) and reliant on a common web browser to render the application executable.
Some examples include: Facebook, Gmail, Bloglines, Twitter, Wikipedia, etc.
Not belonging to this category are news sites, blogs or forums, like for instance CNN.com.