The Flood (sometimes called The Parasite by the Covenant) is a virulent, parasitic lifeform that thrives by consuming other sentient life of sufficient biomass.

The Flood appears in three primary forms: Infection, Combat, and Carrier, but also can evolve into the 'Gravemind' form that appears to act as a kind of hive-mind for the entire species. All of these forms have an attraction to any calcium-based life form of sufficient mass to accommodate them and will try to attack/infect any they encounter.

NOTE: Calcium? Perhaps they meant "carbon".

When the virulent Flood organism reaches a certain critical mass, it develops a centralized intelligence, which calls itself a Gravemind. This is the Flood personified, and it carries with it the genetic memory of millenia of slumber.

The thing was stopped once before by the Forerunner activation of the Halo weapon, but it will not be stopped this time by a weak alliance of ruined humanity or the deluded obsessions of the Prophet of Truth. The Gravemind intends to achieve its ultimate purpose - to consume and absorb every thinking creature in the Galaxy.