(coexisting on another dimensional plane)
History and politics
Millennia before the mid-23rd century, Terrans tended to value cooperation, freedom, and equality, but at some point decided these were dangerous fantasies that inevitably inspired rebellion, and abandoned them in favor of the fascism that eventually defined the species. (DIS: "Vaulting Ambition")
On April 5, 2063, a group of Terrans were filled with fear and suspicion when a Vulcan scout ship landed on Earth to initiate first contact. The Terrans slaughtered the Vulcan crew and used the technology within the scout ship to create the interstellar Terran Empire.
The Terrans conquered many of the races they encountered, including the Vulcans, Orions, and Tellarites. Many species became slaves of the Terrans, but some species, like the Vulcans, were treated better, and could even serve in the Empire's military. (ENT: "In a Mirror, Darkly")
This interstellar empire lasted almost three centuries. In the year 2267, Spock encountered the prime universe James T. Kirk, and became determined to change the empire. Spock eventually became the empire's ruler, and instituted reforms that made the empire much more peaceful. (TOS: "Mirror, Mirror") However, these reforms led to the Terran Empire's downfall; they resulted in the empire's military strength being unable to prevent the equally vicious aspirations of the Alliance of Klingons, Bajorans, and Cardassians from conquering the Empire. When their empire fell, the Terrans were forced into slavery by the Alliance, along with the Vulcans. (DS9: "Crossover", "Through the Looking Glass")
In the year 2371, after an encounter with several prime universe individuals, Benjamin Sisko, a Terran pirate working for the Alliance, decided that he had had enough of slavery and began a revolt. He led a number of Terrans and other species in the Terran Rebellion, a revolution against the Alliance. Though Sisko was killed the same year, the rebellion still lived on. (DS9: "Crossover", "Through the Looking Glass")
A year later, in 2372, the rebels captured the space station Terok Nor above the planet Bajor. The Alliance's ruler, Regent Worf, led a fleet to retake the station, but the rebels managed to drive the Alliance fleet off. (DS9: "Shattered Mirror")
Two years later, in 2375, the rebels captured Worf himself. When last seen, the rebels appeared to be well on their way to achieving victory over the Alliance and winning back their freedom. (DS9: "The Emperor's New Cloak")
"Only compared to a Human from your universe. It's the singular biological difference between our two races."
Terrans and prime universe Humans were physiologically almost identical, since they were exactly the same species, just hailing from different universes. The only immediately noticeable difference was cultural, and even then, it was possible for a prime universe Human to feign bigotry and barbarism and, though more difficult (TOS: "Mirror, Mirror"), a Terran to feign tolerance and decency, meaning an individual could blend in until they were able to return to their home universe. However, there was one physiological difference: Terrans were more sensitive to bright light. This fact led Michael Burnham, upon discovering this, to quickly realize that the Gabriel Lorca she had been serving under had been his mirror universe counterpart all along. (DIS: "Vaulting Ambition")