Growl, is a system-wide notification app, that 'listens' to calls made from Adium or any other application that is registered for use with it.
When a call is made to it from an app, Growl displays in the top-right hand corner of the display. (i.e., missing functionality in Adium, where PC MSN users have a little pop-up that notifies whether someone has come online.)
See
http://en.wikipedia.org/wiki/Growl for more info.
