(covers information from several alternate timelines)
In 2293, the location of Janus VI in the Milky Way Galaxy was labeled in the star chart "The Explored Galaxy", which was on display in Captain James T. Kirk's quarters aboard the USS Enterprise-A. (Star Trek VI: The Undiscovered Country, okudagram)
In 2366, in an alternate timeline, during the Federation-Klingon War, the location of Janus VI was labeled on a tactical situation monitor in the ready room aboard the USS Enterprise-D. (TNG: "Yesterday's Enterprise", okudagram)
A long-established colony, the Janus VI colony contained production facilities that as of 2267 had been in operation for over fifty years. The colony was a major producer of pergium, a dozen planets depending on it. Janus IV had no current volcanic activity, and no atmosphere. Though the planet was rich in mineral resources, reaching them was difficult with standard mining technology. Minerals present included gold, pergium, platinum, cerium, and uranium.
The Federation was unaware of the Horta until that year, when a new level was opened up and the miners inadvertently began destroying Horta eggs, causing the mother Horta to retaliate by sabotaging equipment and killing fifty colonists by the time she was stopped three months later. Once it was realized that the Horta was a sapient creature, an agreement was created so the two sides could share the planet. (TOS: "The Devil in the Dark")
In 2269, when attempting to understand the threat they faced on the Kalandan outpost, after Captain James T. Kirk asked if the rocks could have life, Lieutenant Hikaru Sulu thought it possible that the captain was thinking back to his time on Janus VI and his experience with the Horta. Lieutenant Commander Leonard McCoy disagreed with the captain arguing that the Horta registered as a lifeform and the medical tricorder wasn't picking up life signs from the rocks. (TOS: "That Which Survives")
In 2328, this was the destination for passengers traveling from Denkir Orbital City via commercial transport. Hilary Groner and Robert Levinson traveled there on the SS Kogin. (TNG-R: "Inheritance", okudagram)
The script notes described Janus VI as "an ugly planet... reddish-brown, with a thick turbulent layer of clouds, slowly rotating." Further script notes described it as "hostile looking, reddish-brown, a boiling atmosphere."
For the 2006 "remastered" version of "The Devil in the Dark", a new, computer-generated version of Janus VI was created to replace the stock footage used in so many TOS episodes. New shots of the planet's surface were also created.
According to Star Trek Maps (p. 26), Janus VI was a class-H planet and one of ten planets orbiting the moderate yellow star Janus.
According to The Worlds of the Federation (p. 110), Janus was a class-E planet known as Shaul by the Horta.
According to the RPG sourcebook Worlds, Janus VI was located in the Alpha Janus system.
Janus VI was additionally mentioned in the Star Trek: Deep Space Nine Pocket Books novel Devil in the Sky and, in the IDW comic book A Matter of Perspective, the Enterprise was en route to revisit the planet before being redirected for a distress call from a pleasure cruise.