There are 4 key healing classes in World of Warcraft fighting games : Priests, Shamans, Paladins, and Druids.
Let's look first, at the priest class. Priests include 2 talent trees to select from for healing. The first is the discipline tree and is used mostly for tank healing (even though it can be a very good choice for raid healing depending on the circumstances). They rely chiefly on mitigating damage through the use of instant cast shields. The second tree is the holy tree and whereas they can do very well as a tank healer, they are first and foremost used for raid healing. This tree allows for incredible group heals though 2 spells called circle of healing and prayer of healing.
The second faction of healers is shamans. They only have one tree devoted to healing and that is the restoration tree. These healers are competent tank healers but are used largely for raid healing due to some exclusive talents. The first of which is called chain heal and is a "smart heal" in the sense that it by design targets the lowest level players inside a certain radius. Shamans are greatly sot after in raids due to Heroism (alliance) or Bloodlust (horde).
Next are paladins. Again, they only have one talent tree committed to healing and that is the holy tree. These are straight tank healers due to their incapability to raid heal. However, paladins can often heal 2 tanks at the same time quite successfully due to a spell called Beacon of light which basically allows you to target and heal one person (tank) while automatically healing the other for 50% of the total healed.
The last class capable of healing is druids. Known as healing over time (HoTs) healers, they flourish as raid healers because they can effortlessly keep a raid healed through a lot of damage over time (DoTs). One of their main spells is wild growth which is another smart heal targeting up to 5 raid members inside a certain radius and casting a HoT on them.
Keep in mind that all these classes shooting games boast their uses and are necessary for different fights. All healers can, in certain situations, repeal their roles and become a tank/raid healer.
