(covers information from several alternate timelines)
Khitomer was an inhabited planet located in the Khitomer system. 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")
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 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, amongst 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") An obelisk was erected on Khitomer as a memorial to those killed. (VOY: "Memorial")
Khitomer colonists Edit
"KIT-to-mur" was the pronunciation for this planet's name from the script pronunciation guide for "Redemption". 
The Khitomer hall was represented by the Brandeis-Bardin Institute, located in southern California.
According to the Star Trek: Star Charts (pg. 56), the Khitomer system was located in or near Klingon space, in the Beta Quadrant. This system was a binary star system. Primary was a Class F star with a magnitude of +3, which was ten times brighter than Sol. Secondary was a Class F star. Tertiary was a Class K star.
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 reclaims 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 is the site of a conference between the Federation, the Klingon Empire, the Romulan Star Empire, and the Romulan Republic.