Nooks, like most electronic devices, offer rechargeable batteries inside their devices for ease of use and reduced costs. It is great to know that you won't have to worry about losing power in the middle of a good book, only to discover that you don't have extra batteries lying around. The Nook battery life has gotten better and longer with each model; some can stay charged for quite a while if you are just reading books. Whether you have an old model or a shiny new Nook color, battery options are getting better and better. You can recharge your battery time after time, and when it runs out, replacement is very easy.
Your battery should not need too much in the way of care, other than replacement when it finally drains down. However, if you let your device sit for a long time without use, you should remove the battery and store it in a dry place. If you do let it sit and find that the charge isn't as strong, then you might want to check the connections. A little alcohol on a q-tip or the end of a pencil eraser can be gently rubbed on the battery connections to help clean off any gunk or residue that may have accumulated.
All rechargeable batteries eventually reach a point where they simply don't have enough juice to recharge. This is normal, and you could opt to send your Nook off for service to get the battery replaced. However, it is actually not too hard to install a new battery. You can buy a Nook replacement battery from Barnes & Noble, or you can buy a compatible battery from another retailer. Once you have the new battery, all you need is a Phillips head screwdriver and your own two hands.