"All consciousnes is one, separated only by a thin veil of the physical"
- – Jace Beleren, Vryn's planewalker
"What is a drop of rain, compared to the storm? What is a thought, compared to a mind? Our unity is full of wonder which your tiny individualism cannot even conceive."
- – The Many, System Shock 2
"Ĥ̸̦̘Ẽ̴̻͍͔̜̉͠Y̷̺̌̀͋̚ ̶̼̏G̵̬̬̑̿̒͝Ư̴̲̭̙̠͆̒̅Ȳ̶̧̙̩͌S̸̫̝͎̲̆̈̌̉. ̶̢͙̯̂̓̃̑A̷̺͙̯̟̓̎̃̉Ǹ̴̢͓̀̈́͠Y̵̜̱͚̪͗͊̈́O̶̡̥̾̍͝ͅṆ̶̡̰̓̑͑ͅE̴̘͍̊̀̏ ̴͕͈̓̂H̸̥̘̬̫͋̆U̶̮͉̜̝͌N̵̨̫͗̆̀G̷̩͇̀̕R̶̰̍Y̸̨̩͗̏ ̵̛̱̱̀̅T̷̘͖̂̔̓̐ͅO̵͙͋̎̽D̵͇͗͗Ä̷̫̝̅Y̸̬̑?̶̧͔̬͂̊͐ ̴̪̖͆́͠ ̴͔͍̓̓͌͂B̵̘͓͖͝Ë̶͚̪͇͙́͌C̵͇͉̺͈̈́A̴̲̼̻͐ͅÚ̴̖͓͝S̶̈́̈͜E̴̡̛͔̲̓͘ ̸͚̋W̷̢͗ͅE̶̲̳̰͒̊̆̂ ̷̦̾̀̉S̷̱̻̳̐̈́͠U̸̦̫͕̽̍̉͝R̸̮͌͠È̴̟͠ ̵̢͚̣̻̍͆Á̶̳̗̈́̓͠R̵̪̉̒̒̕E̶̥͍̎̐̕̕.̷̦̎̏̀̈́."
- – Tyranid Hive Mind, TTS
"It is like talking to a herd of hungry sheep. It is unfulfilling and makes you look like an idiot."
You know how your brain is made of a bajillion on/off chemical switches? What if you had a meta-brain that was made of a bajillion brains? Congrats, you're now envisioning what is basically a hive mind. Some hive minds have a central hub 'queen', or multiple 'queens', while others have delegated thinking-drones with their own personalities (Type A). Others however have no sort of "core" units any more than you have "core" neurons (Type B). A hive mind race is depicted as either a race of individuals each in perpetual telepathic contact with all the others, or else, in the logical conclusion of the neuron analogy, the entire race is actually all a single person whose practically in multiple places at once.
Eusocial insects colonies like ants, termites, bees and wasps, are usually the textbook example of a hive mind. Ants, however do not have any dedicated thinking units, nor any command structure: their queens are just immobile baby factories. There is no single thinking body or overriding mind spread among the insects; each individual acts on its own volition, informed by its instincts and acts for the Greater Good of the colony. Said instincts, honed by evolution for social coexistence, gives each individual a predictable purpose and behavior independent of any higher thought organization. Each ant or bee is born knowing its place in the colony, and uses its own intelligence (humans have 344000 times more neurons than an ant) to perform actions that keep the whole colony alive, when said colony is reliant upon every other member of the colony also doing the same to manage resource intake, keeping the hive clean to limit sickness, protect the hive from other predators or rival colonies, and occasionally produce breeding members to make more colonies. Really creepy when you think about it too much.
With a hive mind, you can have from a dozen to a city full of faceless minions that never fail morale checks; because each individual is wholly devoted to serving the greater whole in a Type A hive mind, and because the single person is no more troubled by losing five bodies than you would be by losing five neurons in the Type B hive mind. Given this, their aforementioned connection to vermin, and the fact that the concept easily lends itself to a race of space communist(s) (even though having "core" units would make them a caste system), hive mind races are almost always the villain, being given a raging hard-on for imperialistic expansionism. Cease this instance Guardsman!
Despite this, a hive mind could logically serve several quite nonvillainous roles rather well. Depending on just how the multiple individuals/bodies actually communicate with one another, they could serve as an excellent means of instant long-distance communication, for example. But this is rare in fiction, the closest example of a 'good' hive mind are the Geth from Mass Effect, and they're a loose example given the exact nature of how their intelligence works.
Perhaps the most famous Hive Mind in science fiction is the Borg from Star Trek. A brutally utilitarian and hyper-expansionistic cyborg collective, the Borg are arguably the result of Trekkies learning about the Cybermen from Doctor Who and deciding to splice them with old anti-Communist propaganda. The Borg are a "hybrid" of artificial intelligence, cybernetic components, and captured organic species that have been forcibly linked together through the use of the aforementioned components. Zombie-like, they exist only to spread across the galaxy and assimilate all life into their collective.
Ironically, though firmly created as the type B Hive Mind, the Borg also popularized the Type A or "Hive Queen" variant when the Borg Queen was created as a kind of "face" to put on the collective as a whole.
The titular Xenomorphs were ultimately depicted as having a powerful racial psionic network, with the queens of the different hives as the ultimate leaders of their progeny, in the 80s and 90s comics and novels.
The Zerg from Starcraft may be the most well-known "Hive Queen" style Hive Mind in sci-fi. Though most Zerg average out as "animal" on the intelligence scale, there are distinctly smarter strains of Zerg who can telepathically commune with and control the lesser Zerg as extensions of their own will. This chain of intelligence caps, in the original Starcraft, in the Cerebrates and the Overmind, a singular organism which simultaneously exists within and separate from the collective psyches of the Zerg swarm. Destroying these "ruler-minds" is actually only supposed to be possible in-universe with the use of powerful psionic weapons that can cut them off from the collective mindscape of the Swarm, preventing them from just being downloaded into new bodies.
The Bugs are described as being a eusocial species in the novels. The film turned this into a more sci-fi style Hive Mind, with a distinct caste of psionically adept "Brain Bugs" that literally control the lesser species like puppets, and which in turn bow to the Queen Bugs, which are simultaneously more powerful Brain Bugs and the birthing mothers of the swarms. This interpretation of the Bugs was subsequently ported into the Roughnecks: Starship Troopers cartoon and the D20 tabletop game.
In Warhammer 40k, the "Hive Mind" is the ultimate, supreme grand master awesome badass overlord of all Tyranids. It blots out all psychic signals, including the Astronomican, Astropath communications, and even the Warp. The latter is very upsetting to anyone that isn't a Tyranid (or a Necron, but the majority of them don't care either way).
The Hive Mind is made of literally every Tyranid creature, and the hive fleets are merely appendages of the Hive Mind. This means it is very, very, very big. Fucking huge, and some theorize that the Hive Mind is a new god of Order: its capacity for violence and reach is far beyond that of Khorne's while denying him any actual bloodlust, renders Tzeentch's plans impotent (individual Tyranids have no minds to outsmart), the immortality offered as part of a greater whole is beyond the shambling eternity of Nurgle, and no temptation nor depravity of Slaanesh's can sway a Tyranid away from the Hive Mind any more than you or I could cajole a liver cell into acting like a kidney cell. The Emperor himself probably cannot penetrate the will of an entire spacefaring species acting as one, probably because the Hive Mind is essentially an individual acting through a "species" rather than a collective of minds.
If you want to know its full brilliance: it once outwitted the Ultramarines. There you go.
There are only two recorded cases of the Hive Mind being in any way foiled: Varro Tigurius (in a story written by Matt Ward, natch), and one case when a besieged Craftworld managed to get a distress call out past the Hive Mind to the Eldar pirate prince Yriel (this fleshing out of lore was also written by Matt Ward, but here it was a concentrated effort by an entire planet of highly psychic aliens, and more than one of their best died from the strain).
When facing a Tyranid in battle it is not the individual before you that you fight, but the Hive Mind as a whole operating through the body of said critter. Now this actually is as bad as it sounds, but luckily for the other races of the galaxy the Hive Mind is not flawless. Its largest weakness is that it is pretty much a gestalt, its power and aptitude to influence events growing with the amount of individual bodies in a given place at a given time. There are exceptions to this rule, but generally speaking a single isolated Tyranid organism isn't any more dangerous then, say, a hippo or a wolf. Sure, it can fuck you up; but without higher level thought it can't make a plan other than stampede, gore you and then stomp on the paste. It can't think beyond basic predator hunting tactics and forget about strategic thought. Even a large mass of Hormagaunts, while dangerous, has no ability to form tactics or act with forethought beyond that you would expect from a wolf pack, a wolf pack with swords instead of forelimbs mind you, but wolf pack none the less. But when expressed through the synapse creatures designed to channel it, the Hive Mind can express its full power through its otherwise stupid minions and the Tyranids can become a nigh-on unstoppable unless checked by an equal force, as happened on Octarius against the Orks.
Fun fact, that one actually ended up resulting in an Ork victory, which was kinda refreshing. Nevermind, the Tyranids actually won this one.
Another weakness of the Hive Mind is that even when present in overwhelming numbers, it is neither omniscient nor omnipotent and it still can be tricked, outsmarted or even beaten into submission by sheer resilience and/or firepower. This said: while the Hive Mind does make mistakes and can be outsmarted, you can fool it with the same trick only once; losses are anticipated and are useful either to cull the genepool, as fuel expended to ensure victory and/or simply to gather knowledge about a given lifeform to better adapt against it for the next time a Hive Fleet encounters said lifeform. The Tyranid Codex reminds us that even defeats anywhere may serve to ensure a victory somewhere else -- pray that you are at "anywhere" and not "somewhere else." Again though, for all its power, don't mistake the Hive Mind as more powerful than it actually is. A particularly interesting example of the Hive Mind outright failing can be found during the Devastation of Baal, when, at the height of the battle, the Great Rift opened across the galaxy, and severed Leviathan's connection to the Hive Mind. As in ALL of it. All of Hive Fleet Leviathan cut off from the Greater Hive Mind. And boy, the book makes it clear that the Space-Locusts were NOT ready for this. Even with the deployment of the Swarmlord, it was all the Tyranids could do not to completely lose any remaining cohesion. Then Ka'Bandha and his brodies manifested on one of Baal's moons, utterly upending a critical component of the Tyranids order of battle. And Guilliman's massive fuckoff War Fleets arrived in orbit. And the Legion of the Damned led by the Sanguinor appeared inside the 'Nids ranks to attack them. And Dante killed the Swarmlord. So the Hive Mind regaining some composure didn't quite work out, and, frankly their situation hasn't improved much since due to the impact and fallout of the Great Rift.
Despite the presence of Synapse creatures and Norn Queens, the Hive Mind is much closer to Type B than Type A. You can separate individual Tyranids from the Hive Mind just like a stroke can separate brain cells, but those brain cells don't consider themselves "freed" and will do what they can to link back up.