tenaflyviper:

Oh, gosh.  That’s a toughie, since there are so many different types of horror films out there, and lots of mini subgenres.  For the most part, I think they fall into five main categories:

• Slasher (human vs human):  A slasher like Freddy Krueger pushes the boundaries of the genre by adding a supernatural element, but in the end, the focus is on the body count, and Freddy began life as a human.  The same applies to Jason Voorhees.  Peripherally included in the slasher genre are the Italian giallo (a precursor to the modern slasher), and the German krimi.  These are murder mysteries of a very specific tone (I covered the giallo previously here).  The krimi is essentially a German giallo.  The slasher genre is not restricted just to a single killer, but can include multiple killers (such as in Last House on the Left).  Slashers also include killer kids, though killer babies tend to fall into either the Physical or Creature Feature designations due to the fact that there has to be an unnatural force at work for a baby to successfully become a threat.
• Supernatural (paranormal or religious):  Ghosts, religion, magic, witches, the occult, demons, and possession, for the most part.  Depending on the tone and handling, vampires are typically found here, though there are vampire films that defy this classification (such as Near Dark).  Gothic horror is most often found under this category.
• Psychological (horrors of the mind):  Extreme psychological stress and psychosis.  Mind games, delusions–basically an attack from one’s own psyche, or someone trying to induce psychosis in another.  Often includes amnesia.
• Physical (horrors of the body):  Anything that transforms the human body in an unnatural way.  This includes viruses, extreme surgeries (Tusk; American Mary), scientific experimentation not too distanced from reality (Re-Animator), “melt movies” (Slime City; Street Trash), mutations (From Beyond), and extreme physical torture (Grotesque; Flower of Flesh and Blood).  Zombies are typically represented by this category, but may also be considered a genre of their own due to other interpretations (see below).  This is also the genre that cannibal films call home.  While the conflict is human, cannibal films are not slasher films due to the obvious taboo.  There are slashers that also consumed their victims, but the designation depends mostly on tone and setting (the most popular Italian cannibal films are set in exotic locales), or the emphasis on the cannibalism (a film like Feed would absolutely be considered physical horror).  When it comes to mutations, if they veer too far away from human, they then fall under the category of:
• Creature Feature (non-human adversaries).  This includes animals (of the “nature gone awry” trope), monsters (werewolves, and other creatures of folklore), aliens, giants, and occasionally kaiju.  Human/creature hybrids (The Fly; Creature) also belong here.

Due to their overwhelming popularity, zombie movies may be considered their own category.  While the conflict is human at its roots, zombies usually don’t retain enough faculties to be placed on the same level as slashers, and the greatest threat comes from their numbers.  Many of them fall into the category of physical horror as well, but others (like White Zombie) are more cultural in nature, thus necessitating a distinction of its own.

Then there are the subgenres that cross over with other genres of film in general:

• Science-fiction horror:  Ridley Scott’s Alien is an excellent example of this.  It’s sci-fi, but scary.
• Fantasy horror:  These are most often based on, or reminiscent of, well-known fairy tales, or feature fairy tale elements.  Troll is an excellent example of this.
• Horror comedy:  Humor plays a large part, whether in visual slapstick (Evil Dead II; Dead Alive), comical characters, or intentional camp/cheese (Killer Klowns from Outer Space).
• Horror anthology:  Multiple stories within the same film.  Examples include Creepshow, The ABC’s of Death, and Kwaidan.
• Horror romance:  A somewhat rare category.  Includes films such as A Chinese Ghost Story, Warm Bodies, and Let the Right One In.
• Erotic horror:  Horror with a strong emphasis on sexual content.  Jesus “Jess” Franco was well-known for this particular style.  Quite a lot of vampire films featuring lesbianism can be found here, including Daughters of Darkness.
• Thriller:  Thrillers–while a category of their own–can bear much resemblance to horror.  Giallo and krimi may often be considered thrillers as well.  These tend to encompass murder mysteries, but the level of violence and a tense enough atmosphere can easily make a thriller qualify as horror.

Troma movies, also, may be considered in a category of their own due to
(deliberately) not fitting into only one–or even two–categories at a
time.

All of these categories branch off into numerous subsets.  There are a few designations in particular that can apply across nearly any of these genres.

• Splatter:  Splatter refers specifically to the level of explicit gore, and disregards all other plot aspects.  As long as it contains an absurd amount of grue, it can be classified as splatter.  Psychological horror can contain splatter in the form of physical horror manifesting only in the mind, such as in a film like Jacob’s Ladder (though even so, it doesn’t meet the high level of gore required to make it splatter).
• Exploitation/Grindhouse:  What makes a film exploitation depends on the time period (late 60′s to early 80′s), independent production, lower budget, and most importantly–whether or not there is something to exploit (whether it be nudity, violence, gore, or taboos).  The films of Herschell Gordon Lewis, Ted V Mikels, and Roger Corman can often be found here.  The term “grindhouse” simply refers to the type of privately-owned theaters willing to play exploitation films.
• Horror documentary/Mondo:  As the name implies, this can be a documentary about horror in general, specific subgenres, specific films/franchises, or “mondo” films.  The Faces of Death series is a good example of the mondo film:  An exploitation documentary covering taboo subjects (sex and death), traditions from foreign cultures (usually portrayed with an inherently ethnocentric lens), and staged sequences that are being touted as genuine.  The name originated from the film Mondo Cane.
• Extreme:  This refers to those films that go so far beyond the boundaries of literally everything that they have become infamous.  This is usually due to outrageous subject matter, full frontal nudity, explicit sexuality, social taboos, and excessive levels of violence and gore.  Many films of this nature have seen themselves classified as “video nasties”, the director and crew taken to court for allegations of murder, or have been outright banned in multiple countries.  Examples include Nekromantik, A Serbian Film, The Men Behind the Sun, the Japanese Guinea Pig films, and the August Underground films.

There are also categories that have to do specifically with how the movie is filmed.

• Found footage:  Films that are made to appear as though it is all amateur footage, typically belonging to protagonists that have “disappeared” since filming.  The footage is alleged to be the only thing remaining of the missing characters.  Beginning with Cannibal Holocaust, the genre saw a tremendous resurgence with the popularity of The Blair Witch Project, and is excellently executed in The Bay.
• Shot-on-video:  Exactly as the name implies.  These are typically independent films shot on a shoestring budget, and shot either onto video tape, or digitally.  The majority were produced for the burgeoning home video market of the 80′s.  Because they are rarely (if ever) released theatrically, they are not obligated to remain within MPAA ratings, and thus often contain high levels of gore, taboo subject matter, and strictly no-name casts.  Very rarely, a high-profile film arises that was shot digitally, and thus transcends the SoV subgenre (28 Days Later is the best example of this).  Shot-on-video films tend to be highly unappreciated, but many are forgotten gems just waiting to be discovered.  Both the found footage and SoV genres came together (at least in spirit) in the V/H/S franchise.

The website Horror on Screen created this handy chart, though their classifications and terminology differ a bit from mine:

(Hi-res image can be found here)

The chart uses color to show where there is often crossover between categories.  The site also offers numerous examples of the various lesser categories. 

When it comes to scares/terror, there are really only four kinds:

• Tension/Suspense:  This is the most valuable for a horror film, because this is what keeps the audience glued to their seats.
• Jump Scares:  The cheapest of all scares, these rely on simply catching the viewer off-guard, and startling them.  Anything from a cat randomly landing on the hood of a car, to the killer coming back one last time to grab the heroine’s ankle as she attempts to flee.
• Disturbing revelations:  Usually present in the solving of a mystery, or in an unexpected twist.  Examples include the discovery of previously-unknown parental lineage, and unveiling the murderer.
• Sheer abject horror:  This can be the result of a disturbing revelation.  This is reserved mainly for extreme circumstances.  This is basically how I felt at the very last line of dialogue in A Serbian Film.  It’s that feeling where you’re internally screaming “NO” repeatedly at the screen.

When it comes to effects, the best time period for effects has to be the 80′s, as they were a time when on-set, practical effects were a major selling point of horror.  This is where masters like Tom Savini, Rob Bottin, and the KNB Effects crew carved out their niche.  Many newer horror films rely on a combination of practical and digital, and can blend them to marvelous effect (Slither does a good job of this…mostly).  Splatter films will likely showcase a lot of effects work to accomplish the gore, while non-animal creature features show off all manner of elaborate monsters.  Physical horror is the most intimate, as it deals with the human body exclusively, so it can often be the hardest to watch, and horror that merges closely with science fiction will likely include a cavalcade of intergalactic intruders.  Other subgenres, such as thrillers or milder supernatural horror (old-fashioned house hauntings, for example) may rely solely on suspense and sound design, and not feature much at all in the way of blood.  Other films are quite successful at suggesting a much higher level of blood and violence than they actually show (The Texas Chainsaw Massacre is the epitome of this).  When it comes to exploitation and shot-on-video horror, don’t expect miracles, but do marvel at what can still be accomplished on a low budget.  Ozone is a fantastic example of getting the best out of having little to work with.

That was all quite lengthy, but I hope it helps!