Brotherhood Without Banners

The Brotherhood Without Banners is an organization or group working against Lannister interests in the Riverlands at the time of the War of the Five Kings. The Lannisters claim that they are a group of brigands.

Season 2
At Harrenhal, the Lannister torturer known as the Tickler interrogates prisoners and demands to know where the Brotherhood is and what they are planning.

After the poisoning of Ser Amory Lorch, Ser Gregor Clegane believes the Brotherhood Without Banners is responsible. Lord Tywin Lannister considers the name to be grandiose for a band of brigands and outlaws. Tywin identifies the Brotherhood as a group operating behind the Lannister lines and harrying their forces with impunity, to his annoyance.

In the books
In the Song of Ice and Fire novels a group of soldiers, knights and armed smallfolk known as the Brotherhood Without Banners forms to help protect innocents from the depradations of the war (from both Lannister and Stark forces). By a comparable point in the novels, the Brotherhood Without Banners has become an irritant to the Lannister army at Harrenhal and Ser Gregor Clegane's men are trying to find them.

The "Brotherhood Without Banners" (or BWB) is also the name of George R.R. Martin's fanbase, particularly the part of which gathers around the Westeros.org forums. His fifth Song of Ice and Fire novel, A Dance with Dragons, is dedicated to the Brotherhood Without Banners.