Take, for example, the page you're reading right now. On the web server hosting it, there is an application server that takes article content from a database, formats it, puts it inside some HTML templates, and sends you the results. In this case, the application server is called Kuma. Kuma is built with the programming language Python (using the Django framework). The Mozilla team built Kuma for the specific needs of MDN, but there are many similar applications built on many other technologies.