"Q" is the Federation designation for an impudent, self-superior and sometimes malevolent being from the otherwise mysterious Q Continuum. Beginning in 2364, the alien literally began to pop up in Federation space to tease, torment, and try Starfleet officers — especially Captain Jean-Luc Picard of the U.S.S. Enterprise. From the reports and eyewitness accounts it is not certain that the remainder of his kind share his outlook. It should always be stressed that Q's apparent juvenile humor should never be mistaken for the amoral, unconscionable acts of which he is capable. 
Q is an omnipotent race from Star Trek. See one and you know you're in for (hilarious) trouble. Q's can also be frightening though.