Battery Guru is an app that sits in your menu bar and displays advanced information about the current state of your battery. Battery Guru will monitor and present real-time information on exactly how many milliamps your battery is using at any given moment, and will display it in your menu bar. This way you can keep an eye on the number, and if it gets too high, you know that something is draining it faster than it should be, and you can investigate and close the offending page or application if you're not using it. Your computer comes with a "Smart Battery", which actually already knows a lot of this information and holds it internally. Battery Guru is your means of accessing it, allowing you to monitor the current state of your battery, and to be more aware of what is going on in your system.
The reason your battery sometimes drains quickly is usually due to one of two reasons:
It secondary function is to provide information that you do not usually have access to. It will show you: