Whitelighters are the "guardian angels" of witches and other whitelighters. Mortal charges are not seen to have much interaction with their whitelighters. They watch over their charges and intervene when necessary to help them along their path and keep them safe; they can also be called by their charges. They can constantly hear the lives of their charges in their heads, and if one of them calls their Whitelighter's name, he/she can usually show up at a moment's notice.
Whitelighters are people who were previously somehow connected to the magical world or who were generally good people that had died. They are given the choice to become Whitelighters and given the powers of a Whitelighter (such as orbing, healing, immortality, glamouring etc.) instead of moving on to the afterlife. Their bodies are replicas of their former human bodies, but are composed of orbs which are described as white lights (hence the term Whitelighter) that have a healing sense to them. If struck by a power that would usually kill a mortal (for example, an energy ball or particle acceleration), their bodies will explode into orbs then reform relatively unharmed; they can, however, be knocked unconscious. Because their orbs make their bodies immortal, the only sure way to kill a whitelighter is to poison them with a Darklighter arrow
There is a seeming difference in those who have whitelighter blood in them as opposed to those who were made whitelighters, specifically among hybrids. Many hybrids on the show do not receive knowledge of their charges for some time, and many more are not shown to have any (such as Wyatt Halliwell). Rather, the powers of a whitelighter more or less dignify their heritage.
Whitelighters are the messengers between the Elders and witches or simply the protectors of both inherent and practitioner witches. Leo helps the Charmed Ones by asking the Elders for help whenever they need information not provided by the Book of Shadows.
Healing is another special ability granted to Whitelighters, and is used when their charges have been injured. As long as there is the faintest breath of life left in a person, they can be healed from any injury, whether the injury is magical in origin or not, though they cannot heal self-inflicted damage. A Whitelighter's healing abilitiy is triggered by the emotion of love. As such, whitelighters cannot heal demons or any evil magical beings.
Whitelighters are normally not allowed to have relationships with witches as it is deemed to be too complicated and dangerous by the Elders. There are several half-Whitelighters due to this rule being broken or lifted in special circumstances by the Elders. The four known half-Whitelighters (three in the Halliwell family) are Paige Matthews, Wyatt Halliwell, Chris Halliwell and Sir Simon Marks. All four display most of the abilities of Whitelighters. Wyatt is able to heal, Chris cannot, and Paige apparently can channel Leo's power; during the 8th season, she gains the healing ability for herself. Sir Simon, on the other hand, is a mystery; little is known about him. What is known is that he has the ability to orb himself and he can orb objects with his mind. Whether or not he possesses the ability to heal is unknown. It seems that whitelighter powers, in addition to being granted by the Elders, can be passed on genetically (though they react differently depending on the individual and take different forms).
Because they are pacifists, Whitelighters traditionally avoid fighting, except when desperately needed to protect their charges, but are absolutely not allowed to kill. Leo tries to force Chris into a hearing with the Elders after he suspects (correctly) that Chris killed two warriors in Valhalla, and Leo himself brings balance back into the world of magic by doing a "Great Evil" in killing Gideon.
Among the other powers of Whitelighters are levitation, invisibility, and "cloaking" their charges (making others unable to sense them). Whitelighters can also empathically feel what their charges feel (which, if not controlled, can actually hurt them), knowing if they are hurt or have been killed. White lighters can also summon/create orbs of light. They also have the ability to speak the language of their charges, whatever it may be, and their own sort of glossolalia. Leo is also seen "healing" household appliances, light bulbs, and pipes, and making Phoebe's tea hot again and refilling her water. The power to heal as a whitelighter, at least in Leo's case, is a reflection of his skill as a doctor when he was human and is inherent in his abilities as a handyman. In one episode during the first season where we first realize that Leo has powers, but not that he is a Whitelighter, we see him "heal" the Charmed Ones' Book of Shadows, which had been erased after the Charmed Ones used the spell to relinquish their powers to a Warlock, thus rewriting the entire book and giving back the sisters' powers. Whitelighters also have their own language, which sounds to human ears like a series of clicks and clacks but to them sounds like words. Whitelighters have also displayed the ability to "orb into anyone" which allows them to take the physical form of others. Many tend to confuse this with Glamouring. Glamouring casts an illusion around the being, making it look like another, while orbing into another actually transforms the being into another (more similar to shapeshifting, though in the series shapeshifting is a demonic power). Paige has this ability. While Phoebe and Piper Halliwell have been shown to glamour both themselves and others(in the 7th season finally and at the beginning of the 8th season)), leading us to believe that this is a Whitelighter-exclusive power. Whitelighters also appear to have the ability to mimic certain demonic powers such as energy-balls as Leo Wyatt does when he glamours into the demon Belthazor to train the sisters and as Natalie does while training the sisters to kill a powerful warlock named Eames. Natalie is a fellow Whitelighter and longtime friend of Leo Wyatt during both their Whitelighter years and human years when they fought together in World War II. Elders, whose powers seem to be stronger versions of Whitelighter powers, can throw lightning bolts. Lightning bolts are only used in extreme circumstances such as Leo Wyatt uses when he commits a great evil to restore the balance of good and evil in the two mirroring worlds by killing the Elder Gideon when he tries to kill Leo's son Wyatt. The Elders use them when they try to kill Leo after they view him as a threat after he discloses to them that he is an Avatar.
Whitelighters can pass some, and perhaps in some cases, all, of these abilities on to their offspring. Chris and Paige are able to orb and sense others, channel the ability to heal through another Whitelighter, heal on their own, and speak any language other than their own. Chris cannot levitate, though Paige can. They are, however, still very vulnerable to Darklighter arrows. Paige heals Henry without channeling a Whitelighter's power during an emotional scene that unlocks her power (this could be the natural evolution of her witch powers that the Charmed Ones all have). Until that time, it was thought that half-Whitelighters can not heal, but now it is possible that Chris will eventually gain the healing ability.
Whitelighters have shown a wide range of powers, sometimes appearing when they are convenient and then never appearing again. When Leo explains his powers to Phoebe, he states that they can do whatever their charges need them to do. It is possible, for this reason, that they can only perform certain magics when it benefits their charges, but are otherwise powerless (with the exeception of orbing, healing, and hovering).
The first Whitelighter hybrid ever to appear on Charmed is Paige Matthews. She is the first born Whitelighter and witch. She possesses both the powers of a Whitelighter, which range from orbing to healing, and also witch abilities, such as spellcasting and potion making. It is unknown if she or any of the other Whitelighter hybrids have the power to mimic demonic powers as regular Whitelighters do. Because Paige is of Warren blood and a Charmed One, she also possesses one of the three powers belonging to Melinda Warren. However, since she is also a Whitelighter, her telekinesis became telekinetic-orbing, which is the ability to call for an object and orb it to you. Many more generations of this hybrid will continue in the future.
List of Hybrids:
Note that it is never stated whether Prudence Melinda Halliwell, the third child and only daughter of Piper and Leo, would be a Whitelighter hybrid or not. Leo had became mortal before her conception, so this suggests that he cannot pass any Whitelighter powers onto her, although this is neither denied nor confirmed in the series.
"Whitelighter" is also a term used in the Neopagan community, particularly amongst Wiccans to describe those who see Deity only in the "good", but not in things unpleasant or unappealing. The term used in this sense is generally pejorative.