When creating a character you can choose between six difference classes, Fighter, Mage, Ranger, Shaman, Warrior and Rogue and each of these have multiple roles.
For example, the Mage has two roles, Damage or Healing, and as you level your character you gain skill points that can be used to purchase skills in either of these roles. You may decide to specialise completely in Damage or you might decide to spend points in both.
Upon reaching level 30 you can progress your character into it's 2nd class. These 2nd classes specialise in one of the roles. For a Mage, the 2nd classes are the Magician that specialises in damage or the Priest that specialises in healing. The 3rd and final class is available upon reaching level 75.