Google App Engine enables developers to build their web applications on the same scalable systems that power Google's own apps.
Google App Engine enables you to build web applications on the same scalable systems that power Google applications.

Google App Engine gives you access to the same building blocks that Google uses for its own applications, making it easier to build an application that runs reliably, even under heavy load and with large amounts of data. The development environment includes the following features:

    * Dynamic webserving, with full support of common web technologies
    * Persistent storage (powered by Bigtable and GFS with queries, sorting, and transactions)
    * Automatic scaling and load balancing
    * Google APIs for authenticating users and sending email
    * Fully featured local development environment

Look at following Video for more details