In times long past, in days of honour, mighty men and hardy women would take up arms. These were the Celts of Gaul and Ireland, who would fight not to conquer or kill, but to prove their strength and worth. They would use real weapons and they would die real deaths, but all in all it was just a big contest to prove how great you were.
Bandits is something like that, except we use wooden weapons and nobody dies. Usually.
The premise is simple: Divide into two clans (sometimes more), choose a chieftain, get weapons, then try and kill everyone in the other clan. If you die, you lose your weapons and become part of the clan that killed you. The complexity begins when weapons break, clans can't support new members, chieftains are killed and the bandits come into play.
At any time, with little to no warning, loyal clan members can turn on each other and run away to form their own clans. These are what gives the game its name. Bandits. Volatile, unpredictable and dangerous, the bandits turn their noses up at authority and often have no loyalties at all. Theirs is a life unfettered with the crutch of a chieftain telling them what to do, but one of constant danger.
The rules to Bandits often change with each game, but that is the basic idea. It was created by Nicholas Young a few years back, as a game to play with his five (that's right, five) sisters, and has now morphed into the Event of the Season.