Battle Roles are not an official mechanic, but they do allow for better character choices, when attempting to balance a party.

ROLE(S): What the Hero brings to the table in terms of party composition. This includes Abilities, Actions, and Potions. The roles I can think of (and I'm not an all-seeing seer so if you can think of one I'll add it) are:

AOE: Can deal damage to more than one square at a time. Wave, Sweep, Cross, Lance... all the Area Effects

BLASTER: Capable of high damage rolls against a single target and/or Massive Damage, such as the Riftling Rogue's Backstab. Counter-example (not a blaster!) would be the Hexcast Sorcereress, who without equipment doesn't deal a lot of blasting damage - she's a debuffer.

BUFFER: Can enhance a Hero's rolls or give some Ability that is valuable. The Ember Mage's Fire Water, for example, gives +1B and Fire; the Barbarian's Backlash potion is just downright nasty when thrown at the right target. 

CONTROLLER: Can dictate enemy and friendly movement with Actions and Potions. Compel, Push, and other Control Effects

DEBUFFER: Can impair a Consul monster's rolls or Actions, typically with status effects or Auras. Obviously the Hexcast Sorceress is queen debuffer.

HEALER: Can create a Heal effect by means other than the Bandage Action. Emergency Potions rank higher than Actions, obviously, but any healing is noted. Note Bandage is no longer available in SDE 2.0.

TANK: Can survive above average damage either with increased wounds, defense, or other mechanic. Block hallways, and draw agro/generate Wrath. Abilities like Tough, high Armor, immunity to status effects, and 6+ Hearts all lend themselves to Tanking, as does (potentially) having a large base occupying 4 squares instead of 1. 

THIEF: Can open chests or has other abilities which enhance a party's chance to get good equipment. Luck, high Dex, 7+ Movement, and other variables.