Clerics are holy beings who obtain their powers from the Gods and Goddesses they worship. Their primary goal is to spread the word of their God and convert others to their Faith.
Clerics, regardless of alignment, are also healers and protectors and concern themselves with the health and welfare of the people they come in contact with. To heal and to protect those whom you wish to convert to the "True Faith" is to build a cornerstone for that conversion.
Although most of a cleric's powers are defensive in nature, at higher levels they are granted the ability to use more offensive spells.