In D&D, the cleric is a healer, spellcaster, and sometimes melee fighter. It is a fantasy reimagining of the holy orders of the Knights Templar and Hospitaller of medieval times. As the game has evolved, much of its power now comes through the specific focus of its worship, which is represented mechanically by the Cleric Domain system.
Clerics debuted in Original D&D, and were likewise present as a "Basic" class in BECMI, with the Druid existing as a kind of "prestige class" for the Cleric.
Initially, clerics were restricted to humans, since race functioned as class in these versions of the game. However, subsequent splatbooks would add clerical versions of demihuman or monster PCs; The Dwarves of Rockhome debuted the Dwarf-Cleric. Most non-human clerics were called Shamans, with even Elves and Shadow Elves using this. However, issue #178 of Dragon Magazine featured the Elf-Cleric as part of the serial The Voyage of the Princess Ark.
In 1st edition AD&D, the cleric could wear heavy armor and use a shield. He was forbidden to use weapons that were not blunt, with the only missile weapons being the very feeble sling or the
bizarre staff-sling godly fustibalus. The cleric wore a holy symbol, which he could use to turn away or destroy undead creatures. He could cast spells to mend wounds and cure ailments. He also had spells to protect and buff up the party, some spells to afflict opponents, and some utility spells. The cleric didn't get a lot of ranged attack spells, the exception being the very overpowered Flamestrike. Many of the 1st edition cleric spells were patterned after legendary religious miracles. His excellent armor, good saving throws, and fair hit points made the cleric a decent front-line battle healer. In spite of, or rather because of this, most players didn't want to play a cleric, because most of the time he was the only means of healing, and so he became nothing but a walking first aid kit.
In 2nd edition AD&D, "hybrid" healer classes became more interesting to play. The druid class became more powerful. Several granted powers were added, including the ability to shapeshift into animals. With ranged and powerful attack spells like Fire Seeds, Call Lightning, and The Creeping Doom, the druid could be more exciting to play than the cleric. If you wanted to focus on the undead-battling aspect of the cleric, you could also play a paladin, who had obtained a few upgrades and whose healing was absolutely feeble. 2nd edition also released optional rules for "Priests of Special Mythoi", which allowed you to handpick your spells and new, otherwise unattainable buffs, right from the Players Handbook. By using the rules in the Dungeon Master's Guide for creating a new character class, a player could create a lean but tough priest of a special mythos that advanced in level at a decent pace. In contrast, the cleric only received a buff common to all priests, extra "bonus" spells per day for having a high Wisdom score. Many players chose to play one of those other classes, where their combat clout or attack spells let them be somebody, instead of suffering the insult of being thought of as a reusable healing potion owned by everyone else in the party.
In 3rd edition, a druid can turn into a giant overpowered bear that shoots giant bears out of its eyes while farting lightning and mauling things with its own pet grizzly bear, also overpowered. A cleric can blow all the non-fullcaster members of the party away by self-buffing his stats (to the point where a single hit from his mace will hit with the fury of an angry god's fist) while shooting meta-magicked unresistible fire out of his eyes. They are awesome. Most of the 3rd edition spells were in the 2nd edition spellbook, but there were limitations that made the cleric more of a curiosity than a decent party contributor. But in 3rd edition, if you don't have a cleric or druid in your group, your group tends to die. See: CoDzilla.
Pathfinder reined them in some, and they're now mostly on-par with the other casters if a little more melee-capable than the average wizard. They still get domain powers, but turn undead is now channel energy, which basically lets them heal living creatures and hurt undead in a big radius with positive energy, or vice-versa with negative energy, making them superior healers to almost every other class at every stage of the game. They can also learn to channel those channels into other buffs and benefits too or burn them for single-target damage. They are still awesome.
The 4th edition designers, driven mad by the how absolutely mandatory clerics were for any long-term party, decided to reshuffle roles in an attempt to give everyone the means to survive to some extent without needing a cleric: for the short-term, there are now rules for resting that included non-pitiful amounts of healing outside of a battle. However, when inside a battle, there would be the need for more immediate healing - this is where the "Leader" role came in (Warlords, Bards, Shamans, Ardents, Artificers, Runepriests, and, naturally, Clerics). All classes of this role had a means to provide more on-demand healing, and of course the Cleric was among the frontlines of this.
Even among other leader classes, the Cleric has a very open list of playstyles. Between full-up melee, divine lasers, healing, or something in between, it was easy to have multiple clerics who were dissimilar to each other. Your only constants were Healing Word (your healing power), your cleric lore (Either add your Wis modifier to heals when they spend a healing surge to heal or +2 AC, scale armor, and let you and allies you heal get +2 to attack when you heal them), and Channel Divinity (A once-per-battle power that involved beseeching the gods for something, with feats offering equivalent powers from gods and domains).
After D&D Essentials showed up, the cleric was rebranded as the Templar and a new, non-AEDU System cleric Variant Class, the Warpriest, came into being. The Warpriest had their domains baked into the class' progression, though it's limited in scope with half of them being deities from Forgotten Realms. More than any other class before or after it, however, this class is absolutely crippled in the field of diversity. Once you select a domain, you are forcibly stuck into that domain's specific playstyle.
In 5th edition, they realized that clerics were primarily divine spellcasters the way wizards were primarily arcane spellcasters and decided to throw the clerics some firepower at lower levels, even adding a domain called the Light domain that turns the cleric into a laser-beam-firing machine of death and radiant damage. They also used a nerf bat to break the kneecaps of all the buff spells, making them much less OP (and therefore less aggravating) than they were in 3.5. The end result is clerics that can do whatever they want, thanks to Domains actually doing something now, from healing-focused to investigation-focused to I-wish-I-was-a-druid focused. Oh, and they can use a "channel divinity" class feature with their domain to do cool things now.
|Dungeons & Dragons 4th Edition Classes|
|Player's Handbook 1:||Cleric - Fighter - Paladin - Ranger - Rogue - Warlock - Warlord - Wizard|
|Player's Handbook 2:||Avenger - Barbarian - Bard - Druid - Invoker - Shaman - Sorcerer - Warden|
|Player's Handbook 3:||Ardent - Battlemind - Monk - Psion - Runepriest - Seeker|
|Heroes of X:|| Blackguard - Binder - Cavalier - Elementalist - Hexblade - Hunter|
Mage - Knight - Protector - Scout - Sentinel - Skald - Slayer - Sha'ir - Thief
Vampire - Warpriest - Witch
|Settings Book:||Artificer - Bladesinger - Swordmage|
|Others:||Paragon Path - Epic Destiny|
|Dungeons & Dragons 5th Edition Classes|
| Barbarian - Bard - Cleric - Druid - Fighter - Monk |
Paladin - Ranger - Rogue - Sorcerer - Warlock - Wizard
|Artificer - Mystic|
|The Classes of Pathfinder|
|Core Classes:|| Barbarian - Bard - Cleric - Druid - Fighter - Monk |
Paladin - Ranger - Rogue - Sorcerer - Wizard
| Alchemist - Antipaladin - Cavalier |
Inquisitor - Oracle - Summoner - Witch
| Arcanist - Bloodrager - Brawler - Hunter - Investigator |
Shaman - Skald - Slayer - Swashbuckler - Warpriest
| Kineticist - Medium - Mesmerist |
Occultist - Psychic - Spiritualist
|Ultimate X:||Gunslinger - Magus - Ninja - Samurai - Shifter - Vigilante|