Clerics are servants in the cause of faith and may serve many functions in furtherance of their cause. Clerics can be healers, protectors, monster hunters, guardians, teachers, statesmen or diplomats.  Clerics can likewise be oppressors, liars, reavers, slavers, spies or tyrants.  How a cleric behaves is typified by the demands of his or her faith.

Clerics come from all races and from all walks of life.  Some are ordained into established churches that act internationally and have giant cathedrals and thousands of followers.  Some are passionate devotees to a deity who operate as part of a small congregation.  Others are true believers of a specific religious doctrine, but operate independently of a church hierarchy.  Still others don’t ascribe to a god at all and live their life by a personal philosophy instead of an outside religion.  The one thing that all clerics share is a tangible and potent faith.  Whether it be the existence of a deity or a force such as nature or an established order to the universe or an idea of oneness of spirit among sentient minds, every cleric believes in some force larger than themselves, and it is from their unwavering faith in this force that they draw their power.

At the end of the day, all clerics do what they do to fulfill one of two primary goals.
1:  Win converts to the cause of their faith.
The more believers who follow a faith, the more accepted that faith becomes.  Converting is usually accomplished by spreading dogma teaching the fundamental tenets of a faith.  Sometimes this is enough on its own, but conversion is often aided by performing good works that prove the power of faith. Clerics often perform great tasks like public healing or undergo great quests to bring positive attention to their faith.
2:  Bring glory to the object of their faith.
Living virtuously is often its own reward, but when one’s virtue triumphs over opposition, it brings glory to one’s ideals, church or deity.  If a cleric who espouses health and longevity successfully eradicates a plague, for example, that shows the superiority of his faith over sickness.  Bringing glory in this way is often done as a means of making converts (the aforementioned good works), but is also reward enough on its own, especially to faiths that don’t require masses of converts and worshipers.

It is difficult to pin down the average life of a cleric because there are so many potential cults that he could be a member of. Some are educated from a young age and groomed for a position in a church hierarchy while others are converts that adopt the faith later in life. The one thing that is true of all clerics is that they act according to the demands of their faith.  A good cleric dedicated to a church of healing doesn’t go smiting everyone who doesn’t follow the smallest rule of her church’s dogma.  Her faith requires that she show compassion and understanding, so she might verbally correct someone’s mistake before moving on.  An evil cleric dedicated to a god of plague doesn’t go around stabbing everyone he sees just because he’s evil.  His faith demands that sickness run rampant, which it can’t do if everyone has been stabbed to death, or if he can’t introduce it into the town well because he’s been arrested.

A cleric associated with a particular faith chooses two domains from the list associated with that faith.  A cleric not associated with one of the major faiths may choose any two domains.  As per PHB page 32, a cleric may only select an alignment domain (chaos, evil, good or law) if his or her alignment corresponds to that domain.  A cleric may not cast spells of an alignment opposed to his or her own alignment.  Good clerics channel positive energy and therefore turn or destroy the undead and evil clerics channel negative energy and therefore rebuke or command the undead.  Clerics with a neutral alignment choose whether to turn or rebuke the undead at character creation; once made, this choice may not be changed.