"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..
As a pursuing hostile, Q achieved Warp 9.9 - although that might have been designed to tease the humans; he has since showed capability for instantaneous travel within both space and time within this dimension and perhaps others. The subject Q claims to have an Intelligence Quotient of 2,005 and to be ageless.
The Q being, usually taking the form of a human male, is known to have confronted the El-Aurian native Guinan around 2166, a century before the Borg scattered her people and over 250 years after she first encountered Captain Picard and his crew when they were time-traveling in old San Francisco. Q and Guinan consider each other enemies.
He first appeared to the mainstream Federation when attempting to limit humanity's ongoing advance in the galaxy, charging humans as being a "grievously savage race" and putting Picard's crew on trial as all humanity's representative. Later that year, with the Continuum intrigued by humans, Q tried to tempt Picard's first officer Riker with Q powers so the Continuum would have a human to study. Losing a bet with Picard that Riker would accept them, Q had to promise he'd leave humans alone.
Recalled home for that incident and cast out to wander the galaxy, a bored Q offered to renounce his powers and sign on as a "guide" through the unknown upon reappearing in 2366. Picard's refusal angered him into hurling the ship 7,000 light-years beyond explored space into the initial confrontation with the Borg, where the U.S.S. Enterprise escaped only with his help. Because of such ongoing meddling, Q was reportedly stripped of all his powers by the Continuum and banned again circa Stardate 43539, choosing human form in refuge aboard the U.S.S. Enterprise to hide out from all his enemies. But after selflessly offering himself to the Calamarain to save the starship, his powers were restored.
Captain Picard has reported that Q returned a year later, claiming to still harbor guilt over the unreturned gratitude while arriving unannounced amid a personal matter involving a romantic affair. To apparently learn more about this new vulnerability in Picard, he placed the couple and the ship's senior staff in a Sherwood Forest reality as the legendary Robin Hood, Maid Marian, and the Merry Men - with himself as the Sheriff of Nottingham. After its resolution, the woman, the rogue archeologist Vash, accepted Q's offer for hosted adventures about the universe.
It was this encounter which prompted Starfleet Command to schedule a major service-wide briefing on Q in 2367. Despite his departure with Vash and a year's seeming absence, Q turned up again in Picard's life twice within a few months in 2369, to claim a Q female raised up as a human and to steer Picard on his deathbed through an alternative view of his own life. The busy alien also turned up on Deep Space Nine that year, where he taunted Commander "Benjy" Sisko and his staff.
A year later he sent Picard on a time-slipping trek into three key timelines to help decipher a poser the Continuum had raised to illustrate humanity's future.
Q has called 24th century Earth a "dreary place" and "mind-numbingly dull," preferring it a thousand years earlier with the Crusades, the Spanish Inquisition and - shrinking his own time lag by 400 years - Watergate. However, it was that same boredom that drove him to mischief in the first place.