Google App Engine enables developers to build web applications on the same scalable systems that power our own applications.
Google App Engine lets you run web applications on Google's infrastructure. App Engine applications are easy to build, easy to maintain, and easy to scale as your traffic and data storage needs grow. With App Engine, there are no servers to maintain: You just upload your application, and it's ready to serve your users.
Google App Engine exposes a fully-integrated development environment.
Google App Engine makes it easy to design scalable applications that grow from one to millions of users without infrastructure headaches.
Source:Google