Are they a future human race? Are they an alien race that are masquerading as humans? Are they time travelers? Are they angels?
As of "Face Off", the best guess I can make is that they are both alien and angel. If you listen to The EventCast's exclusive interview with Nick Wauters done in October (you may need iTunes or an RSS reader to listen to it), when asked if Sophia's people are aliens, he said, "it depends on how you define the term alien."
Here's what we DO know...
From episode 2, we know that Sophia's people are "not of terrestrial origin" - that they differ from human DNA by 1%. This may not seem like a lot, but humans and chimpanzees differ by only 2% (according to the show, anyway). From episode 12 ("Inostranka"), we learn from Michael Buchanan that they are from a galaxy called NGC 253. He explains to Leila that life was no longer sustainable so they came to Earth to look for a new home. From episodes 2, 3 and 12, we know that they crash landed on Earth 66 years ago in Alaska. Since they were coming here to look for a new home, one can assume that their plan was to assimilate into our world undetected, and the crash landing and incarceration of the Inostranka detainees upset those plans. Sophia tells Michael in episode 11 ("And Then There Were More") that the Sleepers were not supposed to have children with humans; this is confusing, since if they were planning to assimilate, you would think that was according to plan. Sophia's plan and their goal was to get them home, so perhaps all they were was a scouting party, which Blake Sterling suggests in the same episode.
Also in episode 12, Michael says that they were "found, put in prisons, hunted". This explains the Inostranka detainees, but one can argue that their lack of forthrightness about who they are and what they want resulted, at least partially, in their detainment.
We also know that it is not their way to affect the evolution of our planet, yet Thomas and the Sleepers obviously did so when they assimilated. Some advanced our technology, and made wealth, while others married with humans and started families.
Also in episode 12, Sophia tells Sean that if Thomas is successful bringing more of their people to Earth, "your world will be destroyed." Why? What does that mean exactly? The world as we know it? Is this the end of the world? We don't know yet. Is it the reaction that triggers The Event? Is there something about their people on the planet that is toxic to the people of Earth, such as them carrying a plague that will wipe out humanity, or something else we can't figure out yet? In episode 10 ("Everything Will Change"), Isabel says to Thomas, "Do you know how many of them will die?"
One thing we do know is that, whatever "The Event" is, it does not seem to be stopping them from wanting to bring their people to Earth, especially after episode 14 ("A Message Back"). That would suggest that, whatever it is, it will not affect them.
In episode 15 ("Face Off"), Dempsey shows us an old cave drawing of a giant with people around it, some with eyes and some without.