A Gravemind is a combination of different parasite forms, as discussed in Halo: Combat Evolved. It first starts of as a Proto-Gravemind which connects itself to the surroundings via tentacles. However, it needs a human/other species' body with a sustainable amount of biomass to create a fully fledged Gravemind. Once it has an able body, it will absorb the body's intelligence, make it forget everything and will then shut down the body's major organs which will terminate the body.
It can only work/feed with/on the most intelligent organisms possible, hence the name brain-form. Although these aren't seen much, they are incredibly dangerous and are used for tasks no other can complete. A Proto-Gravemind are sometimes used to pilot a starship, and going back to the biomass section, they will combine their forms with ship officers, pilots and the commanding officer(s) to gather knowledge on how to complete the task. They will use the body's consciousness to gather information out of them via a method known as interrogation. The body is also known as a host. Also, only after the host as been stripped of all knowledge is the host terminated by the Proto-Gravemind. Where it is possible for the host to die before the Proto-Gravemind has assimilated all of the host's knowledge, like has been done before.
A prime example of this is the Truth and Reconciliation, a Proto-Gravemind does exactly this to Captain Jacob Keyes, of the UNSC Halcyon-class light cruiser Pillar of Autumn. If you find the terminal which is associated with this mission, it will give a video showing what the Proto-Gravemind put the Captain through, and you can hear the screams and the noises it used to assimilate his knowledge and make him forget everything. The parasites' intention was to use the Captain's knowledge to pilot the CCS Battlecruiser, as the Gravemind would of had his knowledge to do so. Due to Master Chief Petty Officer, John-117, this action was not completed as Installation 04 was destroyed thanks to the Pillar of Autumn.
This Proto-Gravemind was nowhere near completion. It must grow incredibly in size for it to reach its next stage, the Gravemind, a near-omniscient parasitic form made of thousands of host bodies infused with the knowledge of each host that was used to make the Gravemind. This is formed once a Proto-Gravemind has reached something called a "critical mass" which is where it has developed a centralised intelligence. This only happens on rare occasions, usually because the parasite is seldom loose on a large enough scale to allow Proto-Graveminds to grow large enough to become a Gravemind. Only two Graveminds are known to have ever been created; however, they seem to have the same memories, since the Gravemind present during the Human-Covenant war possessed detailed knowledge concerning the Forerunner-Flood War and even described the actions of the previous Gravemind during that time period as if they were his own. In essence, because it shares all the knowledge of the previous Gravemind and considers them both to be the same being, the Gravemind of the games is the Gravemind that fought the Forerunners, simply been reborn from another Proto-Gravemind.
There is reams and reams of this and I could go on about it all day. Quite frankly, I do not have the energy or the time to do so. Therefore, I will leave it here.