Horror as a genre can be difficult to define and often has significant overlaps with other genre fiction. There is often significant crossover between horror and science fiction, or horror might even be called dark fantasy in some publishing circles. Horror can be synonymous with exaggeration and camp for some diehard fans and film studios. Other times, horror manifests as an exploration of the mundane and unsettling. There is significant crossover here with the thriller genre. So much so, that we've elected to include both genres within this list.
The Horror Writers Association proposes that we take the original definition of horror - the emotion - as "a painful and intense fear, dread, or dismay" and apply that to our genre definition. Horror fiction and media are the things which inspire fear and dread in the person who consumes that media.
So, in defining what horror is, our general guideline is to submit entries that you believe are horror. We understand that this is frustratingly vague. We hope to see a broad and interesting curation of media operating on a vague definition, rather than box ourselves in too narrowly and miss out on some of the best recommendations of the genre.
As we've mentioned above, horror can have significant overlap with other genres. Given that there are other excellent media-based Awesome Lists like awesome-scifi and awesome-fantasy, it can be difficult to decide where recommendations should go.
The maintaners of awesome-horror are not affiliated with the maintainers of the other lists, so we can only speak for ourselves.
If you believe that an entry belongs on both lists, submit it to both lists. Take advantage of the Categories section for your entry to hone your recommendation. Genre definitions are squishy. If you think something is awesome and belongs on this list, we trust your judgement and will not cross reference with other lists for genre purity. :)