True clerics are servants in the cause of a deity. They fufill many functions. The most common goal for most clerics is to win converts to the cause of their faith. This is usually accomplished by spreading dogma, but is often aided by good works, such as public healing. Some clerics perform great tasks or undergo great quests to bring attention to their faith. Likewise, some spread sickness and death because that is what their faith demands of them.
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 the church. Others are converts that adopt the faith later in life. A few even claim to have been chosen not so much by their church as by direct intervention of a deity, having had prophetic dreams or having discovered a holy divine focus waiting for them after some trial. These latter are rare and are often treated with extreme skepticism, even by members of their own faith.
Regardless of how one comes to the faith, the spreading of their ideals is what makes a cleric a cleric. This is not to say that all clerics are preachy (though many are). The most successful clerics often look for unorthadox methods to help shepherd people. Demonstrating the nature of their deity by spreading their message to the smallfolk is among the most common. Examples of this might include a cleric who showcases the magnanimous nature of his deity through good deeds and healing. Another example might be a cleric who attacks and subjegates the public in the name of his cruel god of slavery and tyranny. Another common method is for a cleric to undertake a great quest int he name of his deity. When word of his exploits travels, his name and that of his deity will reach the ears of thousands.
Very rarely, a cleric will arise that does not spread the word of a deity, but the word of an ideal. Such clerics do not normally associate themselves with a cult and in fact, they are often times denounced by existing faiths as heritics and pretenders. These philosopher clerics are a breed apart. Their power is as real as that of any other cleric, but they do not claim that it comes from a deity. Instead, they claim that their power stems from their observance to a particular ideal or lifestyle. Philospher clerics are sometimes popular with the smallfolk because their message of power without the attached strings of a deity is well recieved, but they very often find themselves at odds with other clerics --or worse, with established churches-- that do not appreciate the message that a god is not needed.