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.
Between the end of the Brood War and Kerrigan's defeat on Char, Infested 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.