All clerics have certain powers: The ability to cast spells, the strength of arm to defend their beliefs when the need arises, and the knowledge that their patron deity will listen and sometimes answer their prayers and calls for assistance.
While clerics are not as fierce in combat as warriors, they are trained to use weaponry in the fight for their cause. They can cast spells, primarily to further their god's aims and protect its adherants. They oftentimes have access to only a limited number of offensive spells, but this limited selection can be very powerful.