By default you'll be notified of changes to the power source as well as when your battery loses or gains a charge in the amount of 10%. If you'd prefer to be notifier of every 5%, or maybe 20%, change, you can choose the number.
UnPlugged doesn't do anything Mac OS X doesn't already do itself, as you can check the status of your power and battery in the menubar, but it does do it better. You're likely going to miss an accidentally unplug from time to time and it's nice to be aware of a dwindling battery when you're trying to make use of every minute available. This app doesn't exist for those of you who are satisfied with the little menubar icon, but for people who need something a little more obvious to make sure they don't let their battery run down without their knowledge.
UnPlugged (Free) [Mac App Store]