Watts is a battery calibration tool for Mac OS X computers. To be 100% honest, Watts doesn't really calibrate the battery for you; it simply tells you how to do it and it gives you an array of tools to make the process easier and make sure that everything is done correctly. The app will run all the time and it will constantly tell you the state of the battery, whether it is plugged in to AC power or how much battery you have left. It will also keep a schedule of when you need to calibrate your computer. When the calibration is due, it will guide through the process in 6 simple steps.
To calibrate a battery, you have to fully charge it, then let it drain completely and then recharge it. It is a hardware process as far as I know. If you do it with Watts running, the application will guide you through the process and give you useful remainders, like "save your work", "unplug the AC adapter", etc.
Watts is very nicely designed and it knows about what's best for your battery. It has nice notification and customization options. At $6.95 it is a steal, and you will never forget to calibrate your battery again.