(covers information from several alternate timelines)
Khitomer was an inhabited planet located in the Khitomer system, in the Beta Quadrant. Khitomer played a vital role in the development of the Klingon-Federation alliance, being the site of the Khitomer Conference of 2293 as well as the victim of a Romulan sneak attack on the planet's Klingon colony in 2346, which, in turn, emphasized the importance of the Klingon-Federation friendship. (Star Trek VI: The Undiscovered Country; TNG: "The Neutral Zone", "Sins of the Father"; DIS: "Magic to Make the Sanest Man Go Mad")
In the alternate 2259, the planet was discovered to have a Klingon colony. This event was reported in a news ticker, which was in a collection of graphics and video media that were seen on a powerwall in the offices of Christopher Pike and Alexander Marcus at Starfleet Headquarters. (Star Trek Into Darkness)
In 2293, Khitomer was considered to be a neutral site by both the Federation and the Klingon Empire, and was located near the Romulan border. In that year, the Khitomer Conference was held at Camp Khitomer on the planet. The conference started the first full peace negotiations between the United Federation of Planets and the Klingon Empire and gave birth to the Khitomer Accords. Originally, Earth had been selected as a venue, but after Klingon chancellor Gorkon was assassinated on his way to Earth by members of the Khitomer conspiracy, new chancellor Azetbur relocated the conference to Camp Khitomer. (Star Trek VI: The Undiscovered Country)
By 2346, a Klingon outpost-colony existed on Khitomer. On stardate 23859.7, the colony fell victim to a Romulan attack. In the attack – which later became known as the Khitomer Massacre – 4,000 colonists were killed, among them Mogh and Ja'rod (TNG: "Heart of Glory", "Sins of the Father"). Only two survivors were believed to have been rescued by the USS Intrepid; however, a group of colonists captured by the Romulans were allowed to live and were taken to a prison camp in the Carraya sector, remaining undiscovered until 2369. (TNG: "Birthright, Part I", "Birthright, Part II")
Khitomer colonists Edit
The script for "Sins of the Father" had Kurn state to Worf that the Khitomer Outpost was "new... unfinished" at the time that Mogh visited. 
The Khitomer hall was represented by the Brandeis-Bardin Institute, located in southern California.
The events surrounding the Khitomer Massacre were documented in the The Lost Era novel, The Art of the Impossible.
In Star Trek Online, the Klingon Empire reclaimed the Khitomer sector in 2382 during the political crisis in the Romulan Star Empire that followed Praetor Shinzon's death. In the mission "Turning Point" the planet was the site of a conference between the Federation, the Klingon Empire, the Romulan Star Empire, and the Romulan Republic.
According to the RPG sourcebook Worlds, Khitomer was the third planet in the Beta Guarada system.