Growl is a universal notification utility that Mac programs can use (such as Dropbox,Transmission, Skype, NetNewsWire, etc.). It’s a handy utility that gathers notifications from several applications in one place, so that each app doesn’t have to develop it’s own set of notifications.
Main page for Growl: http://growl.info/about.php
Dropbox is one application that utilizes Growl. For example, if I save a file on my MBP to my Dropbox folder, I will see a Growl notification on my iMac when the file is downloaded and synched on that machine.
I also use it with NetNewsWire so that it tells me when all feeds have been synchronized.
Growl installs as a Pref Pane so you can turn it off universally from System Prefs if you don’t want it. The same Pref Pane also allows you to customize settings for each app. You can also go into your Dropbox Preferences and un-select the option to have Dropbox use Growl.
Here’s a little more info on how Growl works with Dropbox and other apps:http://growl.info/thirdpartyinstallations.php