Modern cars and trucks run on 12-volt electrical systems. 12-volts is enough to get the job done without having so much power that there is danger of electrocution. Today we have more electrical components and do-dads than ever. This really strains your electrical system, making it hard for the battery to keep up. Think about it: electric seats, seat heaters, power locks, windows and sun roofs. And then we have all the power outlets for our cell phones, computers, and DVD players. We also have navigation systems and powerful stereos. Plus there are all the engine and transmission computers, traction control, stability control, anti-lock brakes, sensors and on and on. Even the security system is running off the battery while the car is turned off.
Fortunately, battery technology has given us resilient batteries that are able to meet these strenuous requirements. But the fact is, batteries just wear out over time. Eventually, every battery gets to the point where it cannot hold enough of a charge to start your car. Sometimes batteries need to be replaced because they have just worn out. Or, in other cases, they have developed a leak and need to be replaced.
Special safety precautions are taken when working with batteries in the shop. These precautions also apply to anyone who is poking around the battery. Batteries contain sulfuric acid that can damage your eyes and burn your skin, so safety glasses and rubber gloves are a must. Be careful to not spill acid on your clothes or the vehicle's paint. Of course, avoid short circuiting the battery as well.
Replacement batteries come in all shapes and sizes. Some cars have limited space that requires a specially shaped battery to fit. Larger engines require more powerful batteries to get them started. If you live in a cold climate you will need a more powerful battery because engines are harder to start when it is cold.
Sometimes there is quite a price range for batteries that will work in a particular car. Think of it as "good", "better" and "best". More expensive batteries have a longer warranty and are guaranteed to last longer. As with most things, paying a little more up front saves money in the long run.