In the fictional mythology of "Charmed," whitelighters were a type of guardian angels who would be assigned to particular characters such as the witches in the show to protect and oversee them. Whitelighters had several supernatural powers, such as the ability to instantly transport or "orb" from one place to another, and the ability to heal others under certain conditions. Whitelighters could also be assigned to humans who had the potential to become whitelighters themselves. Those who were overseen by a particular whitelighter were not allowed to know that whitelighter's true identity.
Darklighters were the opposite of whitelighters. Darklighters were assassins who hunted down and killed both whitelighters and potential whitelighters through the use of poisonous arrows. Some darklighters tried to persuade potential whitelighters to kill themselves, thus preventing them from fulfilling their potential. Others concentrated on troubled whitelighters who were on the verge of losing their angelic status. Whitelighters were basically angels, while darklighters were demons.
The whitelighters on "Charmed" were led by elders, who had authority over all whitelighters and their dealings with those they oversaw. Elders had additional powers such as the ability to transport a witch back to a different time. While the elders were portrayed as being more knowledgeable and powerful than either the witches or the whitelighters, they were also depicted as being difficult to work with and reluctant to share information. The elders also had the ability to hear what anyone was saying at any time, giving them in effect the power to spy on the witches.
The whitelighters versus darklighters dichotomy in "Charmed" is a typical example of one of the most common elements in fantasy fiction, in which forces of good and evil, light and dark or order and chaos are locked in eternal struggle for the fate of the universe but are at the same time required to maintain a balance. In the mythology of "Charmed," this balance operates through a parallel universe. If good ever wins the battle completely in our universe then the parallel universe will become completely evil, and vice versa.