Let's start with systemic fluoride in its most common form -- fluoridated water. If a child whose permanent teeth are still forming drinks fluoridated water, the fluoride will make it into her bloodstream and eventually be incorporated into the teeth. It becomes part of the enamel and provides life-long protection from the inside out. That can only happen while the teeth are developing, though -- once they've popped through the gums, they're fully "cooked." But even after that, fluoride from drinking water can help because it becomes part of your saliva, which gives your teeth a constant protective bath.
What about topical fluoride? If you brush with fluoride toothpaste, use a fluoride rinse or get a fluoride treatment at the dentist's office, a little bit of the fluoride will be absorbed by the enamel, reducing the risk for cavities.
Follow your dentist's directions for using topical fluoride to maximize your defense against tooth decay. Fluoride has been responsible for a precipitous drop in the number of cavities, so you should take advantage of it!