There are plenty of stories that are strange, horrific, disturbing, gory, shocking, unsettling, and plenty of other adjectives--but Weird Fiction is its own beast.

H.P. Lovecraft, one of the undisputed masters of the form, wrote an excellent essay on the subject: "Notes On Writing Weird Fiction". If you want to hone your craft, we can't recommend it highly enough.

Here's an excerpt, to whet your appetites:

There are, I think, four distinct types of weird story;
one expressing a mood or feeling,
another expressing a pictorial conception,
a third expressing a general situation, condition, legend, or intellectual conception,
and a fourth explaining a definite tableau or specific dramatic situation or climax.

In another way, weird tales may be grouped into two rough categories—those in which the marvel or horror concerns some condition or phenomenon, and those in which it concerns some action of persons in connexion with a bizarre condition or phenomenon.

Each weird story—to speak more particularly of the horror type—seems to involve five definite elements:
(a) some basic, underlying horror or abnormality—condition, entity, etc.—,
(b) the general effects or bearings of the horror,
(c) the mode of manifestation—object embodying the horror and phenomena observed—,
(d) the types of fear-reaction pertaining to the horror, and
(e) the specific effects of the horror in relation to the given set of conditions.