Jake works on the Google Chrome Team and shares a lot of information about the Application Cache. The AC give you as a developer the possibility to make your web application available offline. You specify what pages will be available offline and they will be cached. It will also reduce server load since the cached content won’t be downloaded again if you online if the server content hasn’t changed.
This is very interesting even though it might be difficult to make an application with a lot of data available offline. Previously I’ve been involved in a project where web server and database was installed on the clients and using database replication in order to make the application available offline. That’s a solution for the complete application to be available but often you don’t need everything and that makes the Application Cache very interesting.
They also get into a discussion about Progressive Enhancement and Jake have some very good posts on his blog about this.