The first broods were formed by the Overmind, with the direction of each brood being delegated to a cerebrate. A brood was engineered for a specific function, which impacted its numerical strength and the diversity of its strains. The terrans gave the Great War broods reporting names based on beasts found in Earth mythology.
A brood depended on a nexus creature to direct it. In the pre-Brood War Swarm, this position was filled by cerebrates. However, supplanting the cerebrate with another entity could allow broods, or other large groups of zerg, to be controlled by non-Swarm agents.
The Swarm was reunited under Kerrigan at the end of the Brood War. The cerebrates were eliminated from the Swarm but broods such as the Char Brood remained.
Between the end of the Brood War and Kerrigan's defeat on Char, Kerrigan created numerous broodmothers, powerful queens, each of which would lead a brood.
The queens were ordered to evolve and only submit to the will of the strongest leader, which became a problem upon Kerrigan's defeat. The brood mothers scattered their broods. Kerrigan was forced to follow and attempt to subdue them.