(covers information from several alternate timelines)
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. (Star Trek: The Animated Series; Star Trek IV: The Voyage Home; Star Trek Into Darkness) The voices of at least some of the females had a soft, cat-like purring quality. Some Caitians did not 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, and were from Cait. 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."
From StarTrek.com, "The felinoid Caitians and the warlike Kzinti share common roots in the deep past — as do Vulcans and Romulans. As with the Vulcans and the Romulans, the two groups went their separate ways to develop as two entirely different civilizations. The Caitians are fierce warriors when so called upon, but pride themselves on their accomplishments in arts and philosophy rather than on the martial arts. Natives of Cait tend to be small and sinewy; the females are as delicately dainty as the Siamese or Abyssinian cats of old Earth. Their family units are extremely close."
The Lincoln Enterprises biography of M'Ress similarly expanded on the Caitians, stating that they tended to be small and sinewy, with their females delicately dainty. 
M'Ress is also referred to as a Caitian in the 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, Shikaris Escort, and Stalker Stealth Fighter. According to their description, Caitians are "known throughout their quadrant for their intelligence, curiosity, loyalty and love of beauty". The game includes several Caitian NPCs, including Enterprise-F Tactical Officer and Security Chief Lieutenant Kyona. It also depicts a species closely related to the Caitians called Ferasans.