The Eight Banners were an administrative division under which all Manchu households were placed.
In the conquest phase, the Manchu armies started with four banners which were later expanded to eight. In these early times, new tribes that were assimilated into the Manchu army were divided over these banners to prevent mutiny. Especially loyal clans were allowed to remain intact and were placed under a single banner. Once the Qing was established, Manchus were born under a certain banner and typically served under that banner for life, although in rare cases entire families were moved from one banner to another.
For an elaborate article, see: Bāqí (八旗)
Chinese: Bāqí (八旗)
Manchu: Jakūn Gūsa