Role

Role is the second half of class design (alongside power source). Where power source determines how you do your thing, role determines just what that thing is. Every class fits into one of the four roles, which are general designations indicating what the class will be doing in combat. The four roles include Defender, Striker, Leader, and Controller, and let the player know at a glance what a given class's specialty is.

Roles make sense from a teamwork perspective. Sports teams and military units alike assign roles because it is efficient. When a group of people get together to accomplish a goal, it is more effective if each person tackles one part than for every person to try to do everything all at once. The four roles this game lists fit the four elements of fantasy combat.

Defender
The Defender role is defined by two abilities: 1) the ability to weather incoming attacks, through high defenses and lots of health, and 2) the ability to draw enemy attacks to them and away from their teammates, who are not as capable of enduring them.

Striker
Strikers provide the main offensive capability in the team. They are the most capable of dealing the most damage, but beyond that, a Striker also needs to be able to decide where that damage goes. Target selection is a big part of what makes a Striker stand out. Since almost every player can do some damage, the Striker simply being able to do more isn't enough: they also need the ability to put that damage in the most effective places, attacking the most vulnerable, highest priority targets. To this end, most Strikers boast excellent mobility.

Leader
Leaders can be charted along two axis: offensive vs. defensive, and proactive vs. reactive. All types of leaders have their place in the game, and they all work towards the same end: enabling their teammates. A defensive, reactive leader probably specializes in healing and granting saving throws, while an offensive, proactive leader applies buffs to attack and damage rolls and grants attacks.

Controller
Probably the most difficult role to pin down, controllers don't apply direct force like strikers, but try to mitigate opposing force.