• There are a little over 20 standard notifications. I'm a big fan of "Achievement unlocked," but you may recognize such hits as "Elle is online" and "Please reconnect controller."
   
• The default position for notifications is in the lower middle of the screen, but game developers can move them to a different place during a game so it doesn't cover up their UI.
   
• There's a little ring of light on the left of each notification. The quadrant that lights up on the notification corresponds to the controller that the notification is for. The quadrant on the controller flashes, too.
   
• Inside the ring of light is a little icon. The icon depends on the notification. For example, when a friend comes online, the icon is the little person ("friend") icon.
   
• Many notifications are "context-sensitive." That means if you press the Xbox Guide button while the notification is up, it'll open the Xbox Guide and jump to the right page. Like, when you receive an achievement, it'll jump to your achievement list.
   
• An internal nickname for a notification is "toast." 
   
• You can turn off notifications (TriXie mentioned this in her blog). To do this, open the Xbox Guide then select Personal Settings, Notifications. Some system notifications will still display, but the setting will persist if you turn your console off and back on.
   
• Another way to turn off notifications is to set your online status to Busy. To do that, open the Xbox Guide, then select Personal Settings, Online Status. This will turn off most (if not all) notifications, but it will reset the next time you turn your console on.

There you go. Notifications in a nutshell. You wouldn't think there was that much to say about the little buggers, but that's what makes this job so dang interesting.