A Birth Plan is pretty much just what it sounds like… a plan written out by you, listing your preferences for an ideal birth. Of course, a “birth plan” is somewhat of an oxymoron, since childbirth is anything but predictable. However, there are certain things you might want to be in control of, and writing out a birth plan for your doctor or midwife will ensure that procedures follow your preferences as much as possible. Of course, if there are (G-d forbid) complications or emergencies, your doctor will decide what is best for you and your baby.
Some things that women include in their birth plans are positions for labor and birth, environment during labor, pain relief options, fetal monitoring, breastfeeding/bottle preferences, and circumcision. However there are many other details that you might want to include. You can write down anything you feel would make enhance your birthing experience, and whatever you think is best for your well-being and the well-being of your child.
If you are interested in creating an interactive online birth plan, BabyZone has a very detailed one. BabyCentre also has some useful information and a list of things to consider when writing a birth plan.