Lovecraft was an American sci-fi, horror, and fantasy author most famous for giving us Cthulhu.
H. P. Lovecraft was an author from the 1920's whose books weren't that widely accepted mostly due to his "challenging the values of the Enlightenment, Romanticism, and Christian humanism," according to Dr. Wikipedia. Because of that fact, like most artists and authors, Mr. Lovecraft didn't get renowned until sometime after his death. On the subject of horror, one of Lovecraft's favorite ways to describe something was to just simply say, "...it look too horrible to describe." Since we don't like to use our imaginations in the Internet Age, those words aren't nearly as appreciated as they were back in Lovecraft's time.
The most notable of Lovecraft's creations would have to be the Cthulhu mythos. Cthulhu is a cosmic entity who first appeared in the short story "The Call of Cthulhu." He/She/It/Whatever is represented with an octopus head, dragon-like body and wings, and a human caricature.
That's a huge bitch!
Lovecraft has had a huge impact in our modern culture. Arkham Asylum in the Batman series is based off of a sanitorium in one of Lovecraft's stories. Not to mention, that 3 of Metallica's songs (The Call of Ktulu, The Thing That Should Not Be, and All Nightmare Long) are based off of Lovecraft's literature.