Healer is a type of character class. Often one of the classes will be designed around the restoration of allies' health, known as healing, in order to delay or prevent their defeat. Such a class can be referred to as a healer. In addition to healing, healer classes are sometimes associated with buffs to assist allies in other ways, and nukes to contribute to the offense when healing is unnecessary.
When both healer and tank classes exist, a common grouping strategy is for the healer to focus healing on an allied tank, while the tank prevents other allies, including the healer, from losing health.
Healers are wise in medicine and apothecary. They know the herbs and simples that can restore vitality, ease pain, anesthetize, and neutralize poisons; and with their instruments, they can divine a being's state of health or sickness. Their medical practice earns them quite reasonable amounts of money, with which they enter the dungeon.
A healer is generally tasked with restoring health, removing poison-like effects, and raising fallen party members. Different games may include different mechanics, such as the ability to deal damage or to enhance the attributes of their allies.
If the tank's job is to take damage, the healer's job is to heal it. In small groups, they may also be tasked to heal the group as well, but in large scale group-play there are typically specific healers assigned to raid-wide damage (typically taken indirectly, via minions or summoned mobs, spells or environment/habitat of the boss).
Tank assignments typically require more high-healing spells over single (or in the largest of groups, 2-3) tanks. It is typically very intense, as letting the tank die can be seen as time wasting for the rest of the group.
The Inquisitor class is capable of healing other characters, but the mechanics are not yet known.