Gravemind is the highest known evolution of the Flood lifeform in the Halo series. A sentient being, the Gravemind is the collective mind of The Flood, and manifests when there are enough flood in the area.
The Gravemind is the intelligence that leads The Flood. The Flood go through three stages, Feral, in which they are disorganised and communicate via means of pheromones, the Coordinated state, in which they grow dominant enough to form a Gravemind, and the Interstellar state, which has to do with the acquisition of technology rather than growth or evolution. A Gravemind is a fusion of pure Flood Super Cells (the building blocks of the Flood), and the neurosystems of one or more intelligent, sentient beings. The Gravemind does not 'lead' the Flood in the traditional sense of the word. The influence it exerts has been likened to a human's control over his limbs. In the Halo series, the Gravemind appears to possess great intelligence, twice tricking the Master Chief and The Arbiter into serving him only to betray them.
It should be noted however, that Gravemind is never explicitly stated to be the leader of the Flood, only that he coordinates them in certain ways.
During the mission "Quarantine Zone" in Halo 2, The Arbiter has an objective of obtaining Delta Halo's Index, the activation key to the Halo's control room. After finally grasping it, the Covenant's other alien race, The Brutes, push the Arbiter into a pit, take the Index and leave him trapped in the dark bowels of Delta Halo. While in this pit, the Arbiter makes contact with The Gravemind, the main intelligence core of the Flood. The Gravemind also has the Master Chief in its grasp, causing both to meet for the first time.