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).