Desktop notifications are deployed to notify users that something important has happened in the “background”. Desktop notifications are now available to extensions in Chrome 5, which means developers can start using the notifications API in their extensions to deliver non-modal messages from web sites. Notifications from an extension have no permission prompts or infobar warnings.
An example of an extension that has adopted desktop notification is the Gmail Notifier extension. See screenshot above. After installing it on your Chrome or Chromium browser, go to the Wrench -> Extensions -> Gmail Notifier. Click on Options and tick “Enable new messages notifications”. Now “Save” and “Allow”. After setting-up your Gmail account, you’ll get new e-mail notifications directly on your desktop. How notifications look and where they’re shown depend on the platform.
To learn how to use the notifications API in your extension, see this documentation.