At some point in the eternal conflict between Law and Chaos one side, or the other, hit upon the idea of having their own supernaturally empowered warrior to use as pawns in the battles. Fighters were helpful, but they would not always have the raw power that these forces would want or need and Magic-Users were useful but they ultimately served their own agendas. This was the origin of the Cleric. Once one side had their own Clerics, the other side needed them as well.
Clerics combine the qualities of a a warrior and a wizard, but into a package that is controlled by Law or by Chaos. The spellcasting ability of the Cleric is entirely dependent on their following the rules of their patrons within Law or Chaos. Not following those rules gets the Cleric stripped of their spellcasting, which can be very dangerous in the types of situations that Clerics tend to find themselves. However, since Clerics are typically chosen from the ranks of the most faithful, breaking these rules is rarely an issue.
Different Clerics fill different roles within an organization, and these roles are represented by Domains. A Domain is a class ability for the Cleric, but one that the player gets to choose. The Domain picked for a Cleric is like a theme for them, giving them purpose within their religion and sometimes within the adventuring group as well. All Clerics get one Domain at first level.
Domains are fairly generic because Clerics tend to fill the same sorts of niches within religions. Groups are encouraged to come up with their own Domains as well, using these as a basis for their own creations. One thing to remember is that these Domains do not work in the same manner as those from other editions of the original game.
Possible Domains
Confessor
One of the central goals of the conflict between Law and Chaos is for one side to win. It is not unusual for one side to use heresies to try to undermine the beliefs of the followers of the other side, with these heresies often causing true believers to unknowingly switch their allegience to the other side. The Confessor can be one who can sniff out these heresies early enough for the group to act and bring their flock back to the right pastures. A Confessor has the ability to Discern Lies in targets a number of times equal to one half of their level, rounded up. The Confessor concentrates on one subject, who must be within sight. Whether or not the subject deliberately and knowingly speaks a lie by discerning disturbances in its aura caused by lying. The ability does not reveal the truth, uncover unintentional inaccuracies, or necessarily reveal evasions. The Confessor also receives a +1 to Saving Throws against illusions.
Inquisitor
The role of the Inquisitor within a religion is to root out and destroy the opposing religions. An Inquisitor is very much the front line of the conflict between Law and Chaos, and must be brave and know that their belief gives them strength against the infidels of their faith. Because of this, the Inquisitor Cleric receives a +1 to Saving Throws against Fear-effects and Illusions. Their faith protects them and allows them to see the Truth.
Witchfinder
Magic-using individuals are powerful allies to the Cleric's religion...as well as powerful foes. It is the Witchfinder who is charged with finding and destroying those magic-users who are against their religion. A Witchfinder has the ability that works like Detect Magic that they are able to use (without casting any spell) once per day, also Witchfinders receive a +1 to Saving Throws against magic.
Aesthete
The Aesthete is a Cleric who has immersed themselves in the study and understanding of the supernatural powers of their religion. They have left their scholarly life to do their part in the eternal conflict between Law and Chaos. Because of their early devotion to study, an Aesthete receives a bonus 1st level spell.
Holy Warrior
Sometimes, the battle is as much about protecting the faithful as it is anything else and the is the role of the Holy Warrior. Holy Warriors receive a +1 to their Hit Dice and a -1 [+1] to their Armor Class. Faith is a strong shield and no one has a stronger faith in their religion and gods than the Holy Warrior does.
Templar
The eternal conflict is war, and it is the part of the Templar to fight in the battles of that war. They are trained and hardened so that they can be ready to be thrown into the deep end of the conflict between Law and Chaos. These are Clerics that hit hard for their faith and use their arsenal to destroy those who do not believe. A Templar gets a +1 to Hit bonus on their attacks, as their gods guide their hand, and +1 to their damage (in a non-magical way).