What we're dealing with
The Fallen - A once-proud alien race, now reduced to the status of pirates and scavengers. Once blessed by the Traveler’s Light, they were eventually deserted (presumably upon the arrival of the Darkness), and have since foraged and stolen to survive. Previously structured by a rigid House system, they’re now consolidated into one group and in disarray. Their suits are filled with Ether, a gaseous substance without which they cannot survive.
Servitor - A Fallen machine that serves as a processing unit for Ether, disseminating it to the Fallen. Larger, more powerful Servitors have been worshipped as mechanical gods.
The Vex - A seemingly robotic civilisation capable of manipulating space and time, in order to travel between different timelines and simulate potential realities for their own tactical advantage. In truth, the Vex are actually the goo that rests inside their machines, the robots themselves merely vehicles. In their natural state, the Vex exist as the kind of white rivers and lakes found on Nessus. Given their use of a vast space-time network, it’s impossible to pinpoint the Vex’s exact point of origin, but there’s belief that the Traveler might have once been responsible for their ascension.
The Vault of Glass - A legendary and ancient Vex installation, and the location for the first Raid in the original Destiny. A semi-religious, semi-scientific location, and the epicentre for some of the Vex’s most mysterious and powerful space-time manipulation activities. Several notable Guardians have been lost to the place, though are not necessarily dead.
The Cabal - A highly militarised, conquest-led civilisation made up of several alien species. The current Cabal are not the original version of this civilisation however, which was once - at least according to one potentially biased account - an affluent realm of decadence and opulent excess.
The Red Legion - An elite subsection of the Cabal, led by Dominus Gaul, a commander envious of the Traveler’s gift to Humanity, who believes that the Cabal’s might makes it the rightful recipient.
The Leviathan - The vast, spacefaring palace that is home to Calus, the exiled, previous emperor of the Cabal, who ruled the civilisation during its gold-adorned party days. Also the location of Destiny 2’s first Raid.
The Hive - A demonic alien race whose origins date back tens of thousands of years. Once apparently a weak, peaceful, and fragile people, they were mutated into their now-hellish form when war and imminent apocalypse saw their leaders forge a pact with an agent of the Darkness, leading to one of the earliest known wars between the Darkness and the Traveler.
The Taken - An enemy faction made up of twisted, Darkness-drenched versions of all of the antagonist civilisations, converted and (previously) ruled by…
Oryx - The first king of the Hive, Oryx (originally Aurash, and originally female), discovered the Darkness along with his two sisters, as the three of them sought a means of survival for their people. The solution was empowerment and elevation through Darkness, which twisted the proto-Hive into the monstrous, occult forms they now exhibit. By communicating directly with the Darkness, Oryx effectively became a representative of it in the material world, and evolved the power to ‘Take’, converting any life-form to part of his Taken army. But we killed him in the first game’s Taken King expansion. So nyerr.
The deep history
Crota - Oryx’s son. He attempted an attack on the solar system in the first game’s Dark Below expansion, but we killed him, which ultimately led to Oryx attacking us in revenge. But they’re both dead now.
Savathun - One of Oryx’s two sisters, traditionally known as the cunning one. She’s been missing throughout the story so far, but the Hive (and Taken) throne is now up for grabs, and there are many harbingers of her return in Destiny 2, particularly on Io. Leaderless Taken perform summoning rituals, sources of Hive magic call her name, and someone has started taking fresh victims.
The Ascendant Realm - The Hive’s ethereal netherrealm, which exists outside our universe, and homes many of the species’ most powerful creatures.
The Books of Sorrow - An ancient tome purported to tell the story of the Hive’s origins. Long story short: Twenty thousand years ago, many refugee races subsist on a churning ocean hell-world called the Firmament. The Traveler plans to euthanise the planet with a tidal wave, in order to wipe out a powerful agent of the Darkness that resides there. Three princesses of one of the weaker races dive beneath the surface in search of a means of survival, following a royal coup, and find the Darkness-aligned Worm Gods. The serpents offer them power in exchange for union with the Darkness, and they accept, becoming Oryx, Savathun, and Xivu Arath, the Hive god-rulers. The first Hive war against the Traveler and its Light-aligned followers would occur soon after.
Worms - Darkness-fuelled parasites, gifted by the Worm Gods, that infest every Hive and act as the source of their power. Every time a Hive kills, the Worm feeds and grows, and makes its host stronger. But the more the worm grows, the more it needs to feed. Thus, the endless, impossible cycle of killing and conquest that fuels the Hive.
The Sky - An old Hive term for Light.
The Deep - An old Hive term for the Darkness.
Ahamkara - Dragon-like creatures that appeared in our solar system around the same time as the Traveler. Powerful, and able to fulfill wishes, they were also a parasitic species - psychically, if not physically - liable to corrupt those they dealt with via the encouragement of selfish, reckless, destructive thoughts. Once sought out by Guardians, they were eventually hunted to apparent extinction when the danger they posed became clear. A relationship to the Worm Gods is suspected, but not entirely understood.
Warmind - A hyper-intelligent, AI supercomputer from the Golden Age, tasked with controlling space-based military systems in order to protect the solar system. A whole network of Warminds once existed across the system, but they were cut off from each other and went dormant during the Collapse.
Rasputin - The Earth Warmind. We awoke him near the start of the first game and reconnected him with the local solar system. He’s since reconnected with Io. Regardless, we know very little of Rasputin’s current perspective on events, or his motivation. He’s long-since reprogrammed his morality and protocols, and has preferred to be left alone since reawakening, inviting us into his compounds only when in need of outside help. There’s also a strong case for Rasputin having shot down the Traveler when it tried to escape during the Collapse, forcing it to stay and create the Guardians.