Priests are a common magical class found in many games that have a class system, very similar to the white mage and cleric.
The priest is a class that can have many roles. The priest will almost always draw power from faith, whether in-universe or through a real system in place in the game.
They may be healers, as well as hard-hitters. They will usually focus on buffing and healing, and may be particularly powerful against "evil" enemies such as undead, demons, etc. Usually, a healing priest will be low in defense and offense, perhaps with the exception of aforementioned enemies. The priest may, however, have high physical attack and use "blessed" weapons quite effectively.
In tabletop RPGs, the priest must usually stick closely to a particular line of faith in order to maintain their abilities. In exchange for this, they are less likely to receive backfire from a particularly strong attack as other magic classes might.
Generally, a high attack is good at countering priests, who will usually have low physical defense. If the priest is a weapon-wielding one, attacking from far away is the best way to remain safe. The player should hit as quickly and as hard as possible to prevent the priest from healing.
If elements are in play, the 'dark' or 'evil' element may be weak to the 'light' or 'holy' element, in which case the player would be at a disadvantage against the priest. However, in some such cases, the 'dark' or 'evil' element may be resistant and strong against the 'light' or 'holy' element, or merely strong against it while still being weak to its attacks. In the former case, the player should likely focus on hitting quickly to take the priest down before they heal, but in the latter case, the player should take more care to avoid being hit by the priest.
If on a team with a priest that focuses mostly on healing and not attacking, the priest should be protected, while the priest buffs the teammates and debuffs enemies, though while continuing to heal as needed.
If the priest has a focus on attacking and healing, they should not need protection. In a game where abilities have limited range, players should stand nearby the priest in case they require emergency healing.
A priest skilled at weaponry may be good at pushing through hordes of enemies, particularly when buffed, as they can heal themselves. Otherwise, the priest should likely focus on healing themselves and finding a team, as the healing priest is most useful with others who can help them fight.
Like the mage, the priest often appears in white robes. They may have religious symbols on them, whether connoting to religion(s) in-game or in the real world, which relate to the religion which they follow and draw power from. This symbol may be reversed if the priest is a shadow priest.
Usually a quiet, reserved, pious soul that gives wise advice to others. If an NPC, they may be found in a church, and may be spoken highly of by those that follow their religion.