For questions about Call of Cthulhu editions up through 6th Edition. (For 7th, use [call-of-cthulhu-7e] instead.)
Call of Cthulhu, by Chaosium Inc., is a role-playing game based on the writings of H.P. Lovecraft and other authors writing using the Lovecraft Mythos.
This tag covers questions about editions up through 6th (published 2004 and earlier), as those are largely intercompatible systems, with each being different editions mostly in the literary sense — a refinement and republishing of the text, rather than a redesign of the game. For questions about 7th edition Call of Cthulhu, which involved greater changes to the game system, use the call-of-cthulhu-7e tag instead.