The Flood is an extragalactic parasitic zombie-like species. They can infect any creature the find with a cell that causes cell division on the host, and uses a powder and spores to infect hosts. They are extremely gruesome, led by a form called the Gravemind. The Gravemind is a mega form of Flood, that keeps the knowledge of every infected host. It serves as the Flood's leader.
The Flood was made by the Precursors in a last-ditch effort to get back at the Forerunners for the Forerunner-Precursor war. They made themselves a powder, so they could infect a host and return to their original forms, but the something went wrong with the powder, and it became the Flood.