A fictional race from the Star Trek universe. They are warriors and depicted as rather fierce and merciless, serving as the recurring villains in both, the different Star Trek series and the movies. They speak the Klingon language.
For further information, see the Wikipedia entry http://en.wikipedia.org/wiki/Klingons.
The guys in the show speak Klingon and are sometimes seen playing Klingon Boggle (0101, 0201, 0207). Sheldon also uses a common Klingon phrase "revenge is a dish best served cold", when he shoots Penny with Paintball (0216).