Halo 2 spoilers! Danger, Will Robinson!
Gravemind is a character in Halo 2, appearing very rarely (in only one major cutscene and a few bits of offscreen dialog) but playing a major role in the game's plot. He never refers to himself by this name; Cortana comes up with it and he's listed as such in the game's voice-acting credits.
What Gravemind is is a highly evolved Flood form- a mass of assimilated corpses of various species repurposed by the parasite into a single entity that looks like the bastard child of a long-dead octopus and the plant monster from Little Shop of Horrors. He's huge, filling most of the space in the chamber beneath the Library where he confronts the Master Chief and the Arbiter, although we never really get a good look at him- just his multipart gash of a mouth, from which foul vapors constantly spew, and swarms of sinuous tentacles. More importantly, Gravemind - unlike the typical Flood cannon fodder - is highly intelligent and possesses a frightening amount of knowledge about his prisoners. He speaks to the Chief and Arbiter in their own languages, and even has a habit of speaking in rhyming iambic pentameter (thanks Toxick for reminding me about this detail). He's familiar with the dogma of the Covenant religion, and at some point in the past managed to subvert and subjugate the Monitor of Delta Halo. He can operate the ring's built-in teleportation network, which itself can reach all the way out to nearby spacecraft, and moves freely despite the Quarantine Zone that contains lesser Flood to the region surrounding the Library.
Thus, Gravemind, as the commander of the Flood, embodies the real danger the Flood pose to the universe, a danger that wasn't really plausible based only on the Flood in the first game. Individual Flood are not much of a threat - disorganized, stupid, barely capable of more than attacking the nearest warm body they can sense. Even large crowds of them can easily be handled without too much trouble by a squad that knows what they're doing. But, give them a collective intelligence that can plan and direct their forces, or allow the clumsy and destructive warriors to perform complex tasks like piloting vehicles or repairing spacecraft, they become an extremely dangerous hegemonizing swarm that justifies the extreme measures the Forerunners were forced to take to counter them. | 
