The templar is a divine warrioran agent and crusader for his god and (possibly) his church. Most are champions of a religious order, using their faith to gird them against the plots of heretics, infidels, and pagans. Others are devoted to a specific holy duty, perhaps one that has been entrusted to their family for generations. Although clerics are servants of their gods and paladins, in different lands they might be called bannerets, champions, gallants, kirks, or warders.
Although different templars express their powers of faith in different ways, they all share the ability to drive themselves further because of this faith. Some of the powers they gain through their unflinching faith are extensions of their normal talents, while others are mystic powers not unlike the divine gifts of clerics and paladins.
The personality and training of a templar is influenced by the dogma to which he subscribes. Those who ascribe to a religion with requirements for charity, chastity, and truthfulness might seem saintlike, while those who worship a care-free earth mother might appear far less chaste and knightlike. Templars conform closely to their faiths ideal, making them stereotypical examples of their culture. Some make no effort for such conformity, however, becoming brooders who do their duty grimly, even when in service to gods of love and joy. Often templars of this kind see themselves as buffers who do the unpleasant tasks required by their position so others need not take on such burdens.
Templars do well as primary front-line melee combatants. Although they lack the rage of barbarians or the broad range of combat skills of fighters, templars powers of conviction compensate well for these lacks. They can function anywhere a fighter could, and might aid party members in ways a fighter cannot. A templar traveling with a cleric of the same faith might well see himself as the clerics guardian, though templars of warrior gods are just as likely to stand aside in combat, to allow the cleric to prove worthiness of their martial deities respect.
Templar vs. Paladin
So, whats the difference between a templar and a paladin? Its twofold, and the first is easy to explain. Although a paladin is an exemplar of the forces of good and law, a templar is an agent of divine will closer in scope and concept to a cleric. Any god can have templars, and chaotic good, true neutral, and even lawful evil templars are just as common (possibly even more so) as lawful good examples of the class. The grim warlord who conquers cities for the god of war, the battlefield hospitaller who escorts healers for the god of healing, and the insane pyromaniac determined to burn the world for the god of madness and fire are all examples of templars, though none could possibly qualify as paladins.
That naturally leads to asking what the crucial difference between a paladin and a lawful good templar is, which touches on the second major difference between the two classes. Put simply, templars are not held to the same standards as paladins, by either their order or their gods. Paladins must follow a strict code of conduct. This code is not just a requirement of their order, its a crucial part of how they gain their power. No matter how reasonable or forgivable a paladins violation of this code might be, it results in a loss of divine power until the paladin atones. If a paladin uses poison to destroy a great evil, even if no other option seemed to exist and the evil had to be destroyed, that paladin loses her divine powers until she atones. A paladin is not even required to worship a god (or similar philosophic concept) as a cleric is. It seems reasonable then to say that its not a god that gives a paladin her power, its the dedication to order and righteousness required of a paladin, and this is why there is no such thing as paladins of other alignments. Only a lawful good character can remain so purely dedicated to a specific set of rules as to gain supernatural power from doing so.