In gameplay, a Healer is a class of character which their primary function is to restore the wounds of others; usually their group/party.
You'd think this is one of the most essential roles that a player can fill in any gaming group, and you'd be right. However, it takes a particular type of personality to want to play a healer since it often requires sacrificing the ability to do other cool stuff with your character and relegating your character to a support role.
This is especially prominent in MMORPGs which require you to take your character down certain skill trees which exclude you from other options. This being a result of the gradual dumbing down of the trinity of classes from Control/Tank/Support with everyone being able to handle damage on their own and healing being a secondary role for support to becoming Damage/Tank/Healing because buffs/debuffs and crowd control were deemed too challenging for lazy developers to balance and then deciding that anything that can take it shouldn't be able to dish it out either. This made fully optimised Healers always in high demand and effective players in the role are broadly regarded as of being higher value than an effective damage dealer. At least until the mid to late 2010s where the trinity was gradually discarded in favour of everything being solo friendly.
In traditional RPG gameplay, a healer need not resign himself to endlessly healing others though. Since pen and paper games require a bit more lateral thinking and are not always bound by the attrition formula of "hit it until it's dead", social interaction becomes much more of an issue and it can be quite common for a healer to become the "face" of gaming groups or at the very least the player who brings the party together, since everyone is ALWAYS going to need a healer at some point so they better treat them acceptably and keep them comfortable. Especially since traditionally, classes capable of providing healing in pen and paper RPGs also offer buffs and debuffs that make it less likely for you to get hurt in the first place and through smart usage of their abilities, could hold their own to some degree if needed, usually at least long enough for someone to bail them out.
Dungeons & Dragons 3rd Edition
The D&D 3rd edition splatbook: Miniatures Handbook gave us a completely dedicated healing spellcaster class from the same vein as video game healers. They can't do much at all except healing.
In fact, like Paladins, Healers are bound by spiritual oaths that make them the closest gameplay equivalent of Stupid Good, since they are required to provide healing to their allies or any other good-aligned creature in need. Otherwise they lose all their class features until they atone. Anyone who chooses this class will also probably find that the only way to play is Stupid Good anyway since there are very few other avenues available to them.
They are bound by similar restrictions on armour as Druids are, so therefore cannot wear metal armour or carry shields without losing the ability to cast spells for 24 hours, meaning they are more fragile than arcane casters and will rely on the support of the rest of the group to stay alive since even Wizards can cast magical defences on themselves in case things get hairy.
While also continuing to compare them to Wizards, Healers have the same BAB progression, making them next to useless in a straight up fight anyway.
While they do have a full divine spellcasting progression like a Cleric does, their spells are almost entirely devoted to healing or removing status ailments, in fact the spellcasting list adds no spells that a Cleric could not already cast! So you might have been better off playing a Cleric rather than a Healer, especially since Clerics can spontaneously cast Cure Wounds spells by converting prepared spells and so do not lose any healing function by attempting to diversify themselves into other roles.
3.X D&D is also very much a game that subscribes to the philosophy of "you need to heal zero percent of the damage you don't take". Healing is strictly inferior to buffs/debuffs that make the enemy less likely to harm you at all. This is further worsened by healing spells very rarely being able to keep up with damaging attacks and of course this isn't even mentioning save or die abilities.
At this point you might be wondering why on earth would you play a healer at all then?
Well, first off you are better at healing, and get to add your CHA bonus to any spell that restores hit point damage, this can mean less need for as many spells to restore the same damage if your charisma bonus is acceptably high enough. Also, once they reach high enough level, their healing spells no longer provoke attacks of opportunity. Meaning they are less worried about getting into the thick of it when trying to heal their allies. Though it's still a bad idea due to armour restrictions.
Secondly, you get a celestial unicorn companion which has a similar progression to a Wizard's familiar with respect to sharing spells and empathic senses, which is probably the coolest feature of the class.
Finally, they get an assortment of spell-like abilities for restorative abilities other than healing... such are removing poison, disease, petrification and the like. Meaning less foresight is required when preparing your spell requirements for the day, contrasted to another divine caster who would have just prepared something more immediately functional at the beginning of the day and just hoped for a situation where it's use becomes required.
Making Healers Work
This class is not really optimal for a player character, since it is so entirely focused on the intended task, considering it came from the miniatures handbook, it is obvious that the class was designed for implementation in pitched battles where they are attached to units.
The class also works nicely as a Cohort character for PCs with the Leadership feat, since it does not require too much development on the part of an NPC. They could even be used as DMPC if nobody wants to play a healer and thus enable the players to do combat part of the adventure without needing to chug health potions like a frat party.
Players wanting to be a Healer should first be steered towards Clerics with the Healing Domain instead, and allowed to play this class only if they are really trying to make a point. In which case the only real way to make it work is to use material that improves their spell list.
One of the more obvious ways to do this is using the rules from the Book of Exalted Deeds which really fits the way this character functions. Most important are the "Sanctified spells," which are able to be cast by any class that prepares spells, which include the healer. A highlight of the Sanctified spells is "Valiant Steed" which summons a unicorn to help you for one year, meaning you can run around with two of the things. Sacred Vow Exalted Feats work well with most of the deficiencies of the class. Such as the Vow of Poverty granting bonuses to you in the absence of any decent armour or weaponry, as does the Vow of Peace which overcomes your complete lack of any offensive options. In fact, a good prestige class option is the Apostle of Peace, which requires the above exalted feats as pre-requisites. While it does hamper your overall Healer spell progression, you do get access to a separate 9th level spell progression which has less spells per day, but a considerably broader range of versatility spells.
The "Dynamic Priest" Feat from Dragonlance: Legends of the Twins allows you to cast spells using your Charisma, reducing MAD.
Other less passive prestige options include the Contemplative from Complete Divine, which aside from a roleplaying requirement involving your GM is quite easy to quality for. It expands the variety of spells available to a healer by adding up to two more domains, as well as granting a number of persistent bonuses like poison/disease immunity, spell resistance and eventually culminating in changing the character into an Outsider.
Divine Oracle from the same book is also easy to qualify for, requiring only the Skill Focus (Religion) feat, since you already know enough divination spells at level one. This class adds some more useful divination spells (through the Oracle domain) and also gives you a measure of protection through mystical Evasion, Trap Sense and Uncanny Dodge to counter your utter lack of defenses from being a healer.
Yet another viable option for Dragonblood characters is the Singer of Concordance prestige class from Races of the Dragon, which makes a Healer even more useful in a group as a support character. It adds a good choice of Domain, expands your Healing ability to even more ridiculous heights by adding d8 to all cure attempts, as well as giving you a supernatural sphere of influence that makes you much harder to attack or interfere with.
While it's Raptoran exclusive and requires you somehow cast Summon Monster IV or Summon Nature's Ally IV (see down), Skypledged from Races of the Wild is potent. It allows you to spontaneously cast any Cleric or Druid spell, albeit at higher cost.
Faiths of Eberron introduces the Domain Icon magic item. These cheapish (10,000 GP) let you sacrifice a prepared divine spell slot to cast a associated domain's spell of the same level or lower. While one must follow a deity with the symbol's domain, this is Eberron material and the Sovereign Host can be worshiped as a pantheon to count as having any domain of a member. An Animal Domain attuned one is the easiest way to get into Skypledged, and its own class features will eventually ensure you stay qualified without always needing to have one. The Rainment of the Four from Magic Item Compendium also lets you sac your spell slots to cast spells not on your list.