A Whitelighter is a guardian angel who protects good witches. They have many powers which include healing, orbing, hearing calls from charges and sensing their charges. Whitelighters also help and protect future whitelighters so that the last ones would be prepared for their destiny. The whitelighters are hired to guide charges by the Elders, although sometimes it happens randomly.
Whitelighters are "guardian angels". 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 have died. They are given the choice to become Whitelighters and given the powers of a Whitelighter (such as orbing, healing, immortality, 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. After becoming a Whitelighter, the person's DNA will have three helices like that of witches and demons, not just two like normal humans.
There is apparently an instruction manual for whitelighters, which is referenced several times, for example, in the season 5 episode "Sam I am" when talking about Paige's whitelighter abilities, Sam says "Didn't you read the manual?" It is also referenced by Leo, when he says the same line to Chris.
Whitelighters have the ability to orb, a kind of magical teleportation. This comes in handy for visiting their many charges around the world, and in special circumstances (as Leo Wyatt states in Season 1 that it is "against the rules") to orb others with them. They can also orb objects around. Orbing is not an exact science and can be disrupted by magic, sending someone to a place they did not intend to go. Whitelighters can use their ability to sense the people they are trying to find before going anywhere.
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. However, this power is channelled directly from the Elders. If the Elders do not wish for someone to be healed, a Whitelighter can do nothing about it. If the person being healed is resisting, the Whitelighter cannot heal their wounds until the person allows them to. A Whitelighter's healing abilitiy is triggered by the emotion of love.
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 three known half-Whitelighters are Paige Matthews, Wyatt Halliwell and Chris Halliwell. All three 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.
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 or see 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. 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 Phoebes's tea hot again and refilling her water. 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 "glamour", which allows them to take the physical form of others. Many tend to confuse this with shapeshifting. Glamouring casts an illusion around the being, making it look like another, while shape shifting actually transforms the being into another. Paige also has this ability.
Whitelighters also 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. While Whitelighters have the ability to mimic demonic powers, they are not allowed to use them unless they are training their witch charges. It is unknown, however, if the demonic powers that they mimic have the same effect that they would if an actual demon were to use it.
Elders, who are essentially powerful senior administrative Whitelighters, 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 have two of the three basic powers of a magical witch i.e. to create and use spells and potions. However, no whitelighter has been seen to have the ability to Scry unless they are also part witch.
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.