On the FAQ Page they have answered the question, What is Google App Engine?
Google App Engine lets you run your 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.
You can serve your app using a free domain name on the appspot.com domain, or use Google Apps to serve it from your own domain. You can share your application with the world, or limit access to members of your organization.
App Engine costs nothing to get started. Sign up for a free account, and you can develop and publish your application for the world to see, at no charge and with no obligation. A free account can use up to 500MB of persistent storage and enough CPU and bandwidth for about 5 million page views a month.
During the preview release of Google App Engine, only free accounts are available. In the near future, you will be able to purchase additional computing resources.
Now that is interesting ... isn't it? Only thing.. that I might not like is it is on Python... and that is 'cause I don't know how to code in Python. If you are interested, then hurry up as the invitation is limited to first 10,000 developers as this is a preview release and I've seen Google's launch cycle... and maybe..in a year or two.. you can expect the Beta..:D naah.. I am sure Beta will come soon... just wait n watch, this is something new Google has brought on table and smells like the beginning of the new Era or web based application development is on its way.. BTW...how about an online Visual Studio?... I would take that one for sure.. :)
BTW, Appspot.com didn't work for me..if it is working for you, please let me know...