Unlike traditional web hosting or self-managed servers, with Google App Engine, you only pay for the resources you use. The application itself does not need to know anything about the resources it is using.
As more people use the application, App Engine allocates more resources for the application and manages the use of those resources. Applications written for App Engine scale automatically. When an application can serve many simultaneous users without degrading performance, we say it scales. In particular, Google App Engine is designed to host applications with many simultaneous users. Of course, a web browser is merely one kind of client: web application infrastructure is well suited to mobile applications, as well. App Engine can serve traditional website content too, such as documents and images, but the environment is especially designed for real-time dynamic applications. By “web application,” we mean an application or service accessed over the Web, usually with a web browser: storefronts with shopping carts, social networking sites, multiplayer games, mobile applications, survey applications, project management, collaboration, publishing, and all the other things we’re discovering are good uses for the Web. Google App Engine is a web application hosting service.