The system uses a plethora of criteria to decide which messages are most important: things like how frequently you open and/or respond to messages from a given sender, how often you read messages that contain a certain keyword, and whether or not the message is addressed solely to you or looks like it was sent to a mailing list. If you come across a message that’s been marked important when it shouldn’t have been, you can hit an arrow to tell Gmail it’s messed up. Likewise, if a message that should have been flagged gets sent to the ‘everything else’ area, you can promote it. Through these actions Gmail gets progressively smarter, so the system should work better over time.
This sounds really cool, I was getting ready to write up how to implement this in Apple’s Mail app and still will, but I can’t wait to try this out.