The Cairn were a humanoid telepathic species who were so far advanced telepathically that they had lost the ability to communicate verbally. Unlike most other telepathic species, Cairn telepathy sent images rather than words, allowing for a much higher rate of information exchange. Due to this fact, other telepathic species had difficulty communicating with the Cairn, though it was possible with training. Privacy was a concept unknown to the Cairn before they started to interact with the Federation.
Ambassador Lwaxana Troi was assigned to help the Cairn integrate with the Federation by both communicating with them telepathically and teaching them to communicate verbally with the help of vocal enhancers. This caused strain on her and that, coupled with working with Hedril, a Cairn youth, brought back painful memories involving the death of her daughter Kestra. (TNG: "Dark Page")
According to the Star Trek: Star Charts, on the chart "United Federation of Planets I", the Cairn were from the planet Cairn, and, by 2378, this planet had become a member of the Federation. According to the Star Trek: The Next Generation - Player's Guide, the Cairn homeworld is called Vair.