You turn the key to start your car and you instead of the expected vroom you hear a slowly more sickening sound followed by the dreaded click...click...click. Even worse, you hear nothing at all. No whirr. No vroom. No action. Chances are the battery isn't getting the juice through the wires to turn the starter. And without that juice you're going nowhere. So how did your battery go dead?
The main function of the battery is to store electricity to start your vehicle. In a perfect world your battery lies in wait, fully charged and full of juice, always ready to kick over your vehicle and get you one your way. While you're driving around the alternator takes over, and along with running all things electrical, sends power to the battery for storage. The alternator sends this extra power into the battery through the battery terminals. One positive. One Negative.
If the battery terminals are corroded, loose or just plain worn out, the battery won't be able to take the charge and will die a slow death until one day, usually when you have to go to that big job interview it will breath its last. A good measure of prevention is to check that battery when you open the hood. Powdery or ugly looking corrosion around the posts indicate terminal continuity problems. It's time to clean 'em up.
There are of course other reasons a battery might go south. A malfunctioning alternator won't make enough juice to charge a battery. A hidden short can drain battery power even when your vehicle is resting. Batteries themselves also have limited lifetimes. If you open your hood and your terminals are tight and sparkling clean suspect these more nefarious electrical gremlins.