The Cairn were a humanoid telepathic species who had no concept of spoken language. 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")
The Cairn were described in the script as "tallish humanoids, intelligent looking – with small protrusions on the sides of their heads that suggest increased cranial capacity." Kirsten Dunst described her character as having "brains on the side of [her] head". ("30th Anniversary Moments", VOY Season 3 DVD special feature)
According to the Star Trek: The Next Generation - Player's Guide, the Cairn homeworld is called Vair. It is the fourth planet in the Visium system. Vair animal life evolved telepathy at a very early stage, so that very species communicate by sound. Due to the unique Cairn method of communication, they had no need to develop a written language and their society developed extremely quickly, going from non-sentient forest-dwelling apes to the invention of warp drive in just one thousand years. The only period of conflict in Cairn history was the Viral Image Wars, when a deranged individual named Rakwin started transmitting disturbing images to others, driving the population insane. The resulting wars nearly drove the Cairn to extinction before an innovator called Wusna developed mental disciplines to protect the mind from violent images. In 2365, the USS Okinawa initiated first contact between the Federation and the Cairn. The contact was assisted by Vulcan and Betazoid officers.