First of all, let me state for the record that classifying things, whether it be music, movies or even vegetables, can become an exercise in futility at times. Is a Giant Panda more like a bear or a racoon? The tug of war among zoologists on this issue has been going on for years.... but what's the point? The fact is, humans naturally like to classify things to help better understand their world. Your brain classifies things whether you like or not, it's part of your biology. So, let's set aside the tired pretention that we don't need to worry about whether a song or movie fits into a certain category. It's in our nature. So, now I again ask the question: what makes something a horror movie?
I think most people's gut reaction to what makes something a horror movie would be: a combination of violence and scares. Given that movies like Friday the 13th, Halloween, Scream, Texas Chainsaw Massacre, Saw and A Nightmare on Elms Street are by far the most successful horror films of the past few decades, this answer isn't surprising. However, this couldn't be farther from the truth. This Venn diagram shows how using violence and scares to categorize a movie as "horror" is just not correct.
The chainsaw scene in Scarface is terrifying and gory, yet no one would ever classify it a horror movie. The "drug deal gone bad" scene in Boogie Nights is both violent and scary, but not even close to resembling a horror movie. So how do we make the distinction?
Wikipedia says "Horror films are movies that strive to elicit the emotions of fear, horror and terror from viewers. Their plots frequently involve themes of death, the supernatural or mental illness." Actually, this may be the best defintion out there. Let's put it to use and see if it can differentiate things using hostage movies as our set...
Okay, so now we can pare down everything and movies like Boogie Nights and Die Hard no longer make the grade. It's got to have fear/horror/terror with themes of the supernatural/mental illness and death.
We've made progress, but are still a long way from determining the "gold standard" - the movie that best represents the genre. W are still left with a large set which includes everything from the Scooby Doo movies to C.H.U.D. Also, borderline movies like Deliverance could arguably still be included. The Associated Content has a helpful article on what makes a good horror film: