The best part of having a laptop is the mobility, which of course requires battery power. There used to be different battery types used for laptops but these days laptops come with a Lithium Ion battery. (also known as Li Ion or LiON)
Unfortunately there is a stigma surrounding rechargeable batteries which do not apply to Lithium Ion, most notably concerning the "memory effect". The memory effect is now widely used to explain why any battery doesn't deliver its full capacity, to counter this people have charged their batteries in certain ways to avoid or try to fix it.
Lithium Ion batteries do not suffer from "memory effect", a poor performing laptop battery may be caused by damage or age which means time for a new one. (doh!)
Some would have you believe you need to fully discharge a laptop battery before you recharge it, which you should NOT do. Lithium Ion laptop batteries have a limited amount of charge cycles(cycle = fully discharged and then fully charged) so if you fully discharge your battery every time before charging then you greatly shorten the life of the pack.
Fully discharging a battery through actual use of the laptop is not a problem because you're actually using the power. Discharging fully just for the sake of keeping your lithium ion battery "healthy" is detrimental.
Battery Life Tips(batteryguide.us)
Does "limited charge cycles" have you worrying about replacing batteries often? Lithium Ion batteries have anywhere from 300-600 charge cycles which is 2-4 years of use for the average user.
To get maximum use out of your lithium ion laptop battery there are a few things to keep in mind.
Make sure the power saving features are enabled when using the laptop on battery power. These features may be disabled manually for many laptops if you absolutely need the maximum speed setting for a task, don't forget to turn power saving back on.
If you are not going to use the battery for a long period of time then try to store it in a cool dry place at about a 40% charge level. Do not freeze!
When using the battery, try to charge it as often as possible as soon as you can.
Don't forget your laptop or battery in your vehicle on a hot summer day.
You should consider buying a spare pack only if you find yourself running out of battery power often, don't buy one to have "just in case". Lithium Ion packs start to age as soon as they're produced and can irreversibly lose around 20% capacity per year even when unused. (note storage tip above to minimize this)