
Tribbles are small, soft, gentle fictional animals in the Star Trek universe whose cute appearance and soothing purring endears them to every sentient race which encounters them — except Klingons. While only appearing in a handful of episodes, they are one of the most popular and famous species in the Star Trek universe.
Tribbles are native to the planet Iota Geminorum IV. They are essentially small bundles of fur with no visible external features. The fur of live-action and animated tribbles ranges from solid white to grey, speckled light to dark brown, yellow to reddish orange, and black. According to Leonard McCoy's dialog their only two purposes in life appear to be to eat and to reproduce, and they perform both of these functions well. McCoy concludes that tribbles use over fifty percent of their metabolism for reproduction. A creature that keeps the tribble population under control on their homeworld is introduced in the animated episode "More Tribbles, More Troubles".
The date of first contact with tribbles has never been established in canon. Chronologically speaking, the earliest known appearance of tribbles was in the 2003 Star Trek: Enterprise episode "The Breach" (which takes place in 2152) in which Doctor Phlox uses them as food for his medicinal pets in Sick Bay. They were presumably known by his race before this date, but most likely were unknown to humans as Hoshi Sato was the only person to see the tribble before Phlox fed it to one of his creatures.
Viewers first saw tribbles in the 1967 Star Trek: The Original Series television episode "The Trouble with Tribbles". The episode was written by David Gerrold, and the original tribble props were designed by Wah Chang. It has been noted as one of the most popular episodes of the series, owing to its intelligent sense of humor. This episode was not, however, a depiction of first contact since it was a human named Cyrano Jones who introduced the tribbles to Captain Kirk's crew, indicating the creatures were known prior to the episode.
In the same episode, Spock explains that Vulcans see no useful purpose for tribbles, so the species is not endearing to them, but it's not mutual for the ever-adoring species. For a moment, Spock (a half-Vulcan) begins to find the charm of the cuddly creatures and is caught lovingly petting one; once Spock realizes everyone is staring at him, he tries to hide his feelings and quickly puts the tribble down, a scene heavily laden with humor, since Spock was just stating how the soothing purr of the tribbles would not affect him: "Fortunately, I am ... immune to its effects," he says, quite obviously affected by it.
Tribbles return in the Star Trek: The Animated Series episode "More Tribbles, More Troubles" and they also make brief reappearances in the bar scene of Star Trek III: The Search for Spock and the evacuation scene of Star Trek: Generations. Although non-canon to the series, some fans speculate that by the time of third Star Trek movie, a genetically-altered breed of tribble may have been created that kept their reproduction in check allowing them to become popular pets aboard starships and stations. Before that time, tribbles were considered a dangerous menace to other planets' environments and transporting it off its native home could incur a lengthy prison sentence.
In Deep Space Nine episode "Trials and Tribble-ations" the crew of the USS Defiant encounter the tribbles once more when they accidentally travel back in time and participate in the events of "The Trouble with Tribbles". Worf reveals that the creatures were actually hunted down and exterminated by the Klingon Empire; specially-trained warriors were sent to kill every tribble in existence, and an armada of Klingon vessels obliterated the tribble homeworld. Klingons are unique among Star Trek's races in their extreme hatred of these cute little creatures (the feeling was apparently mutual). This is because they were "ecological disasters", and emitted a loud shrieking noise instead of their normal soothing purr in the presence of Klingons (this may be due to the tribbles reacting to the Klingons' reptilian ancestry, similar to the tribbles' natural enemy). Deep Space Nine Security Officer Odo dryly remarks, "Another glorious chapter in Klingon history. Tell me, do they still sing songs of the Great Tribble Hunt?" A number of tribbles were brought back to Deep Space Nine from the past and the species was re-established. It is not known whether or not their return was deliberate.
Source: Wikipedia!