Laptop batteries are very particular about the way that they are treated. If you treat them well, you should have no trouble getting a long life from them. However, if you don't charge your battery correctly or if you don't know how to take care of it, it will die long before its time and force you to pay a lot of money to replace it. In order to charge your laptop battery right, you have to start from the first time that you purchase your computer and turn it on. Anyone who buys a laptop should know to turn the computer on and let the battery die completely before charging.
Letting the computer drain itself until the machine actually shuts off will drain the battery completely and get rid of any residual charge so that you can get a fuller charge in the first place. The reason that this is important is because of how batteries work. A battery charges itself and maintains a charge based on what it remembers. If you charge your battery from half-full all the time, it will only remember how to be half-full. Therefore, the battery life will wear down faster than if you get a full charge every time. Once you have an initial full charge, you can begin using your machine.
While some people prefer to leave their laptop plugged in at all times, this isn't always the best solution. For example, if you get into the habit of taking your laptop here and there and always plugging it back in as soon as you are done, you'll do a lot of partial charging, which will wear out the battery a lot faster. However, if you always let it run down completely, or at least 90% of the way before you charge it, it will retain a much better battery life.
If you do leave your laptop on the charger all the time, you've got nothing to worry about. Your computer will be running off of the AC power, allowing your battery to remain full and rest in case you ever need it. For a lot of people, laptops are used on the go and proper charging is a must. However, for those who are buying desktop replacements, use through the AC adapter might be the best solution. Whatever you do, make sure that you don't get into the habit of partial charging if you want to get the most from your laptop battery.