The Flood are an extragalactic 'parasite' (Term coined by the idiotic Forerunners) led by the poetic Gravemind. As Guilty Guilty explained to Johnny, the Flood spread its infection via sentient spores, called Infection Forms, that infect hosts and transform them into mutated creatures of surprising strength and agility. When the host grows weak, they are transformed again into bloated Carrier Forms which incubate new Infection Forms to begin the cycle again.
Not only that, the Flood 'Super Cell' can bind pure flesh together and give it new life, these are called Pure Forms. Each Pure Form is composed entirely of pure Flood Flesh.
The Gravemind is a pure form, but he also has a collective intelligence derived from every single Flood organism. This allows him access to a vast pool of knowledge that is constantly expanding, giving him a serious God complex and massive database of vocabulary to form into poetry.
The Flood were originally created by the Precursors as a means of assimilating every race in the Galaxy together to form one massive collective, thus eliminating pain, famine, disease, death and war. The Flood were created as the ultimate life form that would consume the Galaxy and create a super-race, but obviously some people fought back against this threat, and the Forerunners were the first to create an 'effective' countermeasure for the Flood - the Halo Array. Despite the fact that the Flood were annihilated by this stupid tactic, the idiocy of the Forerunners meant that they rose again, and it was left up to the Humans and Elites to put them back in line.
Despite all this, the Flood are actually a peace-loving race. They only aimed to assimilate all life into one species to prevent racial conflict and differences, and they were taught the error of their ways by The League and convinced they could best keep the peace by serving in the League Military. Today, the Flood serve as one of the strongest supporters of League policies.