Basically it turns Chrome into a desktop notifier for Gmail, so you don’t have to run any separate notifier if you’re big on getting the heads up when new email (or chats) arrive. Clicking a notification will either open up the new email in a new window or direct your back to your Gmail tab (which is what happened in the video above).
To turn them on, click on the Settings link in the top right corner of Gmail and scroll down to the “Desktop Notifications” section. If you just want to get notified about chat messages, or if you use Priority Inbox and only want to get notifications for important messages, you can customise your settings from there too.
Clearly this is also a good thing for Chrome OS, which isn’t capable of running regular background apps, but can background web apps—and display their notifications. Spicy.
The new notifications ship with the option to toggle chat notifications and choose between notifications for any new mail, or just for Important mail (that is, email that hits your Priority Inbox). Right now the notifications are only available for Chrome users, but Google says they’re working on bringing them to other browsers as well.
Use the new feature wisely. Nothing kills a productive work session like another pop-up notification. You can also adjust the way notifications work—including the position they appear on your screen—by clicking the wrench icon on the notification itself.
Desktop notifications for emails and chat messages [Official Gmail Blog]