An overuse of obscure words and description overkill. In writing, this often results in the piece becoming utterly incoherent and annoying to no end.
This is most often carried out by pretentious dipshits who are trying to sound intelligent, but actually sound intelligent to no one but themselves.
If someone ever uses more than 5 adjectives to describe the colour green, they are said to be one hella annoying Lovecraftian pig-fucker.
Said of a creature, it means something that merely knowing it exists can wear away at ones sanity, and seeing it can be even worse. Such creatures are usually older than humanity, from another planet or dimension, or both. They are not, in the classical sense, evil, but view humans as humans view ants. Stereotypical Lovecraftian creatues are slimey with lots of tentacles.
Said of a story, it can mean several things:
1) A story where the protagonist stumbles upon Things Man Was Not Meant To Know, and ends up telling the story either from within an insane asylum, or from a journal he kept that is discovered after he mysteriously disappears.
2) Any story containing a Lovecraftian creature.
The monster from "The Thing" is pretty Lovecraftian.