My impression from the posters, trailer, and the producers (who also did Paranormal Activity and Insidious) was that this was probably a demon possession film. It's not; instead, aliens.
Don't worry, that's not a spoiler; it's easily guessed early on. Alien films are a difficult one to pull off: your tone has to be exactly right (Signs is a good example of what happens when it goes wrong). Dark Skies takes a fairly decent shot at the genre and doesn't do too badly.
Lacy (Keri Russell) and Daniel (Josh Hamilton) are an ordinary family trying to make ends meet, but events take a turn for the worse when strange things start happening in the night. Their youngest child, Sam (Kadan Rockett), blames "the Sandman" for the shenanigans. Things escalate, complete with three flocks of birds flinging themselve at the house, with the family becoming increasingly frightened and the neighbours increasingly suspicious.
After some internet sleuthing, Lacy discovers that her family is not alone - and apparently, humanity in general isn't alone either.
There are some good character moments in this film; a lot of the time, horror films throw away characterisation in favour of jumpscares, and while that has its place it does mean that you don't particularly care if someone dies. In Dark Skies, the two parents are ordinary people, believable and fairly sympathetic - they're dealing with the recession and one child beginning to go through puberty and act out. It's a solid depiction of a family that begins to be torn apart from the inside (as well as rejected from outside), and a good set-up for an ending where they all have to stick together. It's just a shame that the actors are mediocre.
The tension is done very well, with - as in Paranormal Activity - a particular sound which announces the presence of the supernatural beings and grows in intensity as they grow closer, more powerful, or more hostile. It's spoiled a little by an early reveal of what the aliens look like, but then for the rest of the film you're looking out for them to pop up somewhere else.
There's a fairly obvious twist at the end and a slightly pointless epilogue scene, but all in all this is a fairly solid film with some good tension and shocks. 7/10
Watch the trailer here: