Wikipedia: Cthulhu Mythos

Cthulhu Mythos is a generic name, coined by the writer August Derleth, for a particular type of horror story by the writer H. P. Lovecraft, or one of his followers, which deals with the fictional pantheon of the (so-called) Old Ones. These were immensely powerful beings that once ruled the Earth, but the universe changed (in some unspecified way, e.g., perhaps Planck's constant altered) such that they could no longer live in it. Many fled the old universe, awaiting another change in the universe, the day "when the stars are right" and they can return to claim what they believe to be their rightful inheritance. The high priest of the Old Ones, Cthulhu?, remains on earth in the sunken city of R'lyeh, beneath the Pacific Ocean, in a death-like sleep. From there, his thoughts influence the dreams of his half-human worshippers, who work towards the day when [Cthulhu awakes]? and the Old Ones return. At the present time, they can be called to this universe, but only for brief periods, usually leading to the death or madness of the principal character of the story.

The Old Ones arouse fear in humans by their sheer wrongness--they simply do not belong to this universe--even the geometry of the buildings of R'Lyeh appears subtly non-Euclidean, and the Old Ones themselves are adorned with numerous tentacles and suckers; Cthulhu is a titanic protoplasmic mass of vaguely humanoid form, though with many cephalopod?-like qualities.

The Cthulhu mythos was first described explicitly in the story "The Call of Cthulhu"; there is a role-playing game using the Mythos sold under that name

There is also another group of these alien beings, whom the Old Ones fear, known as the Elder Gods, who can sometimes assist in the fight against the Old Ones, but they were more an invention of later writers, particularly August Derleth.

Lovecraft's followers include writers such as Derleth, Robert Bloch, Clark Ashton Smith, [Algernon Blackwood]?, [Frank Belknap Long]?, and others who have written in a similar vein.