(covers information from several alternate timelines)
The feline appearance of Caitians included long manes and a tail. While the skin of some Caitians was covered with fur that varied in color from orange to black, other Caitians were without fur or trimmed down completely. Female Caitians grew long Human-like hair on their heads. (Star Trek: The Animated Series; Star Trek IV: The Voyage Home; Star Trek Into Darkness) The voices females speak with had a soft, cat-like purring quality. Some Caitians did not find it necessary to use footwear. (Star Trek: The Animated Series)
The name of the species comes from a biography of Lt. M'Ress, published by Lincoln Enterprises in 1974, which claims that the Caitians are related to the Kzinti. This is corroborated by extensive commentary by Larry Niven, author of most of the Known Space books and virtually the entire Known Space sub-series The Man-Kzin Wars, primarily found in his short story books "N-Space," "Playgrounds of the Mind," and "The Patchwork Man."
The Lincoln Enterprises biography of M'Ress expanded on the Caitians, stating that they tended to be small and sinewy, with their females delicately dainty. Their planet, Cait, was described to have atmosphere, mass, and gravity all near Earth normal. There was only one other planet in the system, and it was uninhabited. 
According to the Star Trek: Star Charts, on page 37, the Caitians were from the Cait (15 Lyncis) system. This system's primary was a large F-type star in the Alpha Quadrant. Shane Johnson's The Worlds of the Federation, on page 38, adds that the Caitians inhabited the system's second planet, the only inhabitable one in the system, which they called "Ferasa" and which had two moons, known indigenously as Rea and Sura; Johnson adds, on the same page, that the 15 Lyncis system was the twelfth Federation member to ratify the Constitution of the United Federation of Planets and its component Articles Of Federation.
M'Ress is also referred to as a Caitian in the non-canon audio story "Passage to Moauv".
Set in 2409, the computer game Star Trek Online features the Caitians as a playable species of the Federation faction, which also employs ships of unique Caitian design, e.g., the Atrox Carrier and the Stalker Stealth Fighter. The game includes several Caitian NPCs, including Enterprise-F Tactical Officer and Security Chief Lieutenant Kyona.