Tribbles are small animals know for their reproductive capabilities (they are born pregnant), their mutual dislike of Klingons (they admit an unpleasant sound when in close proximity to Klingons), and their ability to overrun their environment.
Star Trek: The Original Series
In the Original Series Tribbles are brought aboard the USS Enterprise
while the crew is transporting grain in an episode titled "The Trouble With Tribbles". They soon overrun the ship and eat most of the grain and proceed to die. This leads Kirk
and the crew to discover the grain has been poisoned and to discover the culprit is a Klingon in disguise.
Star Trek: The Animated Series
Tribbles make their return to the animated version of Star Trek in an episode titled "More Trouble, More Tribbles". The canon of the animated series is suspect but in this episode the Tribbles have been genetically altered to grow larger instead of reproduce but still manage to overrun the Enterprise
Star Trek: Deep Space Nine
In "Trials and Tribble-ations" some of the crew of DS9 travel back in time to prevent the assassination of Captain James Kirk. Captain Sisko
and this team arrive during the timeline of "The Trouble With Tirbbles" and stop the attempt