Web sites began as information pages, static informative pages usually in html (hyper text mark-up language). As time has gone on websites have developed into web applications. Moving from static pages to interactive pages, ones that interact with user by input or browser, OS and location data to produce a website on the fly. The ability to operate an application online has been an important move forward and enables all sorts of useful applications form Office 365, email services like Gmail, ecommerce sites, social media sites, video content players, messaging services and many many more.
The downside to this is the requirement of the web application to take an input from a user or browser and act on or interact with this information in some way or other, as well as the applications requirement to have a database, information and management systems to operate.