Brigands are humans similar to Bandits, but who have clearly been lost to chaos and evil. Rather than simply self-interested, brigands are actively cruel and destructive, sociopathic, amoral, and contemptuous.
Their unfliching wickedness and way of life gives brigands a surprising amount of courage -- they are far less likely to run from a fight to preserve their own lives, prefering to inflict suffering on their enemies for as long as possible.
Large brigand groups are lead by fighters of significantly high skill, spread fairly evenly throughout the hierarchy. The brigand leaders occasionally work together with skilled wizards and clerics of deities related to thievery, violence, or bloodshed (the clerics tend to be a bit weaker than the wizards or fighters in the force).
Brigands outfit themselves well as warriors, with horses (in flatter lands), swords, shields, chainmail and leather armor, spears, bows, and crossbows. Generally speaking, brigands are separated into mounted melee and ranged groups, and infantry melee and ranged groups, with the melee groups further broken down by swords or spears. Most of the brigands tend to be infantry, armed with swords and shields.
Brigands usually dwell in cave complexes, in ruins, or in fortified structures they've build (or comandeered) themselves. Occasionally, they will have simple camps. Brigand lairs often have captives and camp followers such as merchants (noncombatants).