Bandits are a group of humans (or, occasionally, other races) that live outside of the law. They waylay travelers, harass farmers and merchants, and may even take over entire towns, if left unchecked. Bandits are self-interested above all, and vary with the level of cruelty they impose: some just want to make a living for themselves (if in a way outside the law), while others delight in cruelty and violence. Most bandits tend to be outside the law, but organized fairly well within their own clans (leading them toward Neutral or Neutral Evil alignments).
Large bandit groups are lead by fighters and theives of significantly high skill, spread fairly evenly throughout the hierarchy. The bandit leaders occasionally work together with skilled wizards and clerics of deities related to thievery, violence, or outcasts (the clerics tend to be a bit weaker than the wizards or fighters in the force).
Bandits outfit themselves well as warriors, with horses (in flatter lands), swords, shields, chainmail and leather armor, spears, bows, and crossbows. Generally speaking, bandits 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 bandits tend to be infantry, armed with swords and shields.
Bandits usually make small, informal, mobile camps. Occasionally, they dwell in cave complexes, in ruins, or in fortified structures they've build (or comandeered) themselves. Bandit lairs often have prisoners and camp followers such as merchants (noncombatants).