"Nearly every application on your computer these days needs to notify you of something at some point. Your IM client tells you your best friend just signed on. Mail tells you that an important email from your boss for a status request has just arrived. Toast informs you your CD is done burning. Currently, every one of these notifications arrives in a different manner. Some are audible, others visual, others a mix, or else just a message somewhere you can check when you get to it."