What's the Difference Between a Great Old One and an Outer God?

The two tiers are not always kept distinct in the witnesses' own wording — the classification this Archive uses owes as much to later correspondents who compared the accounts side by side as to any single chronicle — but the working distinction this catalog applies is genuinely useful, and worth stating plainly.

Great Old Ones

Physically real, currently dormant or imprisoned, and tied to a specific place. Cthulhu is the clearest example on file: a vast entity presently sleeping in the sunken city of R'lyeh, worshipped by scattered cults, capable of influencing dreams even while dormant, but not — per this catalog's own findings — an active, roaming threat in the way his fame today might suggest.

Outer Gods

Cosmic rather than local, near-omnipotent, and generally indifferent or actively hostile to being understood in human terms at all. Azathoth, the "blind idiot god," sits at the literal center of infinity as pure mindless chaos. Yog-Sothoth is described as coterminous with all time and space rather than located anywhere in particular. Nyarlathotep is the one exception with anything resembling individual will or purpose — filed as the Outer Gods' messenger, and the only one of the four who regularly takes a form that can hold a conversation. Shub-Niggurath rounds out the set as a fertility-associated entity referenced more often than directly shown.