Circle of Healing is a level 100 Holy priest talent. It is an instant-cast spell that heals up to 5 nearby party or raid members within a 30 yard radius of the target. It does not necessarily heal the person you cast the spell on.
Prayer of Healing has no cooldown and so can be cast as often as required.
Circle of Healing's instant capability not only prevents enemies from interrupting the cast, but means it is not affected by pushback, unlike [Prayer of Healing].
Without improvements, Circle of Healing is 5% more efficient than Prayer of Healing, therefore eclipsing it in terms of speed, HPS and HPM. In these cases Circle of Healing should be used instead of Prayer of Healing whenever possible.
Circle of Healing is a major contributor to the impressive burst healing ability of the holy priest. The spell can be used directly following a [Prayer of Healing], healing 5 targets at virtually the same time (6 if glyphed).
The heal will not be a major hitter alone, healing for low amounts of healing on individual targets.
Being a "smart heal", the spell does very low amounts of overhealing. Many priests end up spamming the spell on cooldown due to it pushing priests high on healing meters as a result.
In TBC and in early WotLK, the dominant healing style of the Holy priest was to spam this spell mindlessly. The massive amount of healing done by "CoH-spam" trivialized a lot of encounters, which led Blizzard to add (and later increase) the spell's cooldown.