Healing Types
This is more simply-put than the DPS Types. To keep other players alive you either undo damage done to them, or prevent it from happening in the first place. Healers typically have multiple types of healing abilities. Some of them are slow but mana efficient, allowing you to manage your resource to keep people alive while also keeping you from running dry.
They will also typically have some emergency spells. These are very mana inefficient, and should only be used when there is a sudden need for them. This sacrifices efficiency in order to improve a situation going downhill very quickly. Sometimes this is worth it to keep people alive, because being mana inefficient is worth it if the alternative is having lots of mana, but your team is dead with you soon to follow.
Shielding your allies from harm is also applicable, but rarely are there whole playstyles that revolve around this. This includes putting up barriers that reduce or eliminate incoming damage, using CC to keep enemies busy, or increasing ally’s resistances to certain types of damage.