As it turns out, a Gravemind is born when a Flood reaches a certain critical mass. It is the Flood personified, and carries with it the genetic memory of millenia of slumber. The purpose of a Gravemind is to consume and absorb every thinking creature in the Galaxy.
Only one Gravemind has manifested before the Halo 2 Gravemind. The first was stopped by the first activation of the Halo rings, by the Forerunners.
The Gravemind is a collection of fallen Flood and other lifeforms.