The paramedics of Norrath.
The Templar is a good cleric that specializes in healing, particularly in keeping a main tank alive through serious incoming damage. Along with their evil counterpart the Inquisitor, this class has an ability to let their "overhealed" damage apply after their target receives more hits. That means that they waste far less mana, particularly in raid settings when healers generally try to continually keep a stream of healing going towards the tank to prevent any burst damage from bringing them down. Their buffs chiefly raise the defenses of their target, making them important for increasing the survivability of tanks and other characters that expect to be taking damage during a fight.
Templars chiefly need to focus on wisdom, with equipment that directly increases power after that and stamina and agility following those. Many of the Templar's best abilities are very power intensive, which makes it important that they have enough to keep up with the damage hitting their group. They also can use a variety of utility heals, such as Vigilant Benediction, which occasionally activates to absorb melee hits. Involuntary Healer also grants a chance to heal and cure trauma on every melee hit. The Reverence spell grants a chance for an ally to heal damage every time that they use an ability that costs power. Sanctuary is a very important spell that renders the entire group highly resistant to stun and mesmerization spells - a crucial ability in some situations, especially when facing spellcasting enemies with crowd control spells. Their Redoubt and Symbol lines of spells greatly improve the hitpoints of any target that has it - an important buff that the priest should keep up at all times.
The Templar can also use some divine direct damage spells primarily effective against undead, but otherwise they will mostly focus on healing their group. Despite wearing heavy armor, they don't make for very good solo classes because they lack a lot of ability to do damage. That's quite alright, as they will be constantly in demand for their healing speciality. They also have some short term mesmerization and stun spells that can be used for some light crowd control, but not really as a dedicated role. It makes for a good backup for when more monsters come in with a pull than the group can handle. The Templar does have some group heals, but those are best used only sparingly, as they're very mana expensive and will draw a lot of threat towards the healer.