Does Tolkien ever mention or describe them in any of his other work?
But the bit about them hating the sunlight the same as trolls and orcs, makes me think they're supposed to be a somewhat more fantastic race.
Maybe they hated the sunlight because they lived primarily in caves or underground, like Morlocks. That could explain their hatred of sunlight.
JRRT might have mentioned something more about them in his Letters, but I doubt it.
There they had been mustered for the sack if the City and the rape of Gondor, waiting on the call of their Captain. He now was destroyed; but Gothmog the lieutenant of Morgul had flung them into the fray; Easterlings with axes, and Variags of Khand, Southrons in scarlet, and out of Far Harad black men like half-trolls with white eyes and red tongues. RotK - 'The Battle of the Pelennor Fields'
Also those half-trolls may have been Olog-hai, who could endure the Sun similar to the Uruk-hai.
The O.E.D. notes (thanks to Hammond and Scull) that in the old Russian chronicle of Nestor Variags is used of the people usually named Varangians 'Scandinavian rovers who in the 9th and 10th centuries overran parts of Russia and reached Constantinople' (note also the Varangian Guard of the Byzantine Emperor).
According to Webster's Third New International Dictionary of the English Language: variag n. cap Russ Varyag of Scand origin; akin to ON Vaeringjar Varangians: Varangian.