The Cthulhu Mythos is a term used to describe a whole genre of stories that are united by references to the same characters, places, books or creatures, many of these creatures being worshiped as gods. In particular, Cthulhu is the most well-known of these gods.
Cthulhu, and many of these other elements, were created by noted early-20th century author H. P. Lovecraft in his tales of "cosmic horror", but Lovecraft generally used and reused names and places in his works simply to provide the illusion of a deeper background story, and is thought to have been unconcerned with the notion of creating a master continuity into which all these crossing-over details could be fitted snugly and consistently. Most of what we now know as the 'Mythos' – as a coherent mythology – was pieced together by his acolyte August Derleth after his death and further codified in the Call of Cthulhu popular role-playing game.
An All-Encompassing Mythos
The Cthulhu Mythos, amongst the wealth of material that has been encompassed by the term, can be argued to include The Hastur Mythos and possibly The Carcosa Mythos, terms kept distinct here because the latter is the main concern of this wiki, being focused on The King In Yellow (both the play and the entity) and The Yellow Sign. Hastur, of course, is a part of that, but 'he' also manifests in other ways – the name is given a much greater identity within the Cthulhu Mythos, which raises the question of whether it is still compatible with its origins in the earlier stories. The Cthulhu Mythos also contains other mythoi unrelated to The Carcosa Mythos, such as the Severn Valley Mythos of Ramsay Campbell.
The Role-Playing Game
The Call of Cthulhu RPG has been one of the more coherent guides to the Cthulhu Mythos, in that it attempts to create a structure to the Mythos, to establish connections between different factions, and to present them in a way that helps rather than confuses. Many of the terms and clarifications about elements contained within the Mythos are due to this game, and its numerous supplements.
Major Elements of The Cthulhu Mythos
The following gods are perhaps the most prominent gods known through the Cthulhu Mythos, being much like universal forces rather than figurative god-like figures. They are known as the Outer Gods, a definition popularized by the Call of Cthulhu RPG.
Several major entities fit into this category:
Great Old Ones
There are many alien gods worshiped by strange cults in this fictional universe. Many of them are worshiped as part of a pantheon of gods, as dictated by locality and beliefs. Cthulhu is the most infamous of these beings from the stars, known collectively as the Great Old Ones (again, a definition popularized by the role playing game), but there are many more.
Here are a few of the more widely-recognized ones:
Books and Artifacts
For more on the Cthulhu Mythos, see the H.P. Lovecraft Wiki here on Wikia.