The horror genre is so VAST that many things can be classified as such. Where theorists come into play, things like Alien are almost always categorised into the horror basket, with sci-fi as a sub genre. This is because it has the codes and conventions of sci-fi, but the over-riding story is one of suspense, terror, paranoia and body horror (if you're interested in reading anything on it, Barbara Creed writes tonnes on this stuff). A lot of the time horror is categorised as male fear of the feminine, whether that be through body horror (like, the man giving 'birth' to the alien in Alien), a 'phallic' weapon as a penis substitute or extension (see: Slasher genre) or 'the horrific mother' figure (see things like Psycho, Friday 13th, etc) just to name a few. Anything that plays with fear ultimately can be called horror. If something is scary throughout, it can be labelled horror. And 'scary' covers a LOT of ground, including disgust, feelings of suspense, paranoia, flinching in pain (like when we see someone stabbed, cut, etc), etc.
Freud (yay) writes about something called the 'uncanny'. This is basically the main breakdown of why we find things scary (again, if you're into horror films, read it, it really gives you a clear view of fear and the horrific). He basically, like Barbara Creed, says that horror is a genre which MEN are afraid of because it demonstrates the horrific nature of woman. Sex is seen as horrific (and punishable) in these films because it represents castration (the penis disappears when it goes inside someone, no?) and that is ultimately man's worst fear: loss of manhood. And all instances of 'horror' can be traced back to this primal fear, even if it is not explicitly demonstrated in a sexual way (like the 'phallic' weapons, etc).
It really is quite remarkable =]