A web application is an application that responds to requests over the Web. Typically, these requests come from a user’s web browser, when the user types the URL of your app or visits a link or bookmark, or when your app’s JavaScript client code makes requests of its own. Requests could also come from other clients on the network, such as mobile or desktop applications, or systems accessing your app as a service.

To build an App Engine application, you write code for one or more request handlers, and describe to App Engine which requests go to which handlers, using configuration. The life of a request handler begins when a single request arrives, and ends when the handler has done the necessary work and calculated the response.