This particular sub genre is probably the most widely known, certainly on the big screen. Movies like Alien and its sequels, the Predator films, Event Horizon, and the like, have helped propel Horror SF into the limelight.
Most Horror SF stories will include a hostile alien race determined to wipe out humanity. Of course it can go the other way to, for we all know that humanity is capable of unspeakable cruelty.
Not all stories within the sub genre fit into the above description. Many thrillers, such as the movie Sunshine, fit into the Horror SF sub genre.
Horror SF sometimes works best on a psyhcological level, the suggestion of horror, rather than horror itself. In those cases, Horror SF can be combined with Soft Science Fiction.