People who tap into Divine Magic actually channel the innate creative will of a Divine, be it a Dragon Divine, or according to some rumours, another being of the same power. Divine Magic is a gift from the Divine(s), awarded for exceptional service and dedication. It is what differentiates a common priest from a true Cleric, or a valorous knight from a true Paladin.
Clerics are the servants of Divines. They have dedicated their lives to their tenents and have sacrificed their time and other things as necessitated by the canon of their god. The type of magic is somewhat similar to Arcane, but with different weight and intent.
Paladins are the warriors and champions of their Divines. While Clerics are there to nurture and keep the traditions going, Paladins follow a stricter code and are the martial will of their chosen Divine. Typically they roam the world, righting (as they perceive) wrongs and bringing battle to the threats to their Divines.
The Druids are a group of people who rise to care for the war-torn lands in the Age of War. Druids channel the magical powers of nature spirits, who in turn channel the power of Mishra and Sithrakh, whose purpose is to handle nature and life.