Mister Mxyzptlk is an antagonist of Superman comics and media. He is a classic example of a trickster villain, as unlike many of Superman's enemies, he does not seek conquest or ruin. Rather, he just wants to mess with Superman for amusement as well as teach lessons to others, albeit in the manner of a trickster-god. He served as the main antagonist in What Ever Happened To The Man Of Tomorrow and Supergirl: Cosmic Adventures in the 8th Grade.
In the TV show Superboy, he was played by the late Michael J. Pollard. In Superman: The Animated Series, he was voiced by Gilbert Gottfried, who also voices Iago from Disney's Aladdin.
Mister Mxyzptlk is a trouble-making imp from the Fifth Dimension who has been harassing Superman since the early days for no reason other than boredom and slight malice - his actions often get him arrested and set to trial in his own dimension but he continues to return since punishment doesn't really affect a being like Mxyzptlk - who can just as easily escape any imprisonment or sentence he may have had imposed on himself.
Superman first met Mxyzptlk when he appeared in Metropolis as the charismatic Ben DeRoy. Mr. Mxyzptlk once forced Superman and The Flash (Wally West) into a race that The Flash won. Mr. Mxyzptlk encountered Lex Luthor and learned the concept of how to lie. Later, Mr. Mxyzptlk supplied Luthor with red kryptonite which made Superman's powers disappear, but Luthor violated Mr. Mxyzptlk's conditions and so Superman regained his powers. Mxyzptlk appeared in Metropolis and tried granting a wish to everybody in the city. He even tried, albeit unsuccessful at the time, to reunite Superman and Lois Lane.
There is a time when even a trickster is out-tricked. Mxyzptlk paid a visit to the insane Clown Prince of Crime, The Joker, at his cell in Arkham Asylum. He toyed with the notion of setting the Joker free with 1% of his power and watch Superman and Batman dance to his tune. Smarter than he may have seemed to Mxyzptlk, the Joker tricked Mxyzptlk into revealing his imp name and gained power over him. He took 99% of Mxzyptlk's power and used it to proclaim himself Emperor of a world he reshaped in his image. Superman, finding the flaw in his twisted logic realized that to the Joker, his existence is nothing without Batman. He couldn't even erase his own memories of the Dark Knight and therefore his reality was thwarted and Mxyzptlk was able to reign his powers back in and return to the 5th Dimension
Mxy has done many other things and he got bored of tormenting humans and other beings weaker than him, so when he found Superman, he was thrilled.
Powers and Abilities
Mxyzptlk is a virtually omnipotent reality warper due to extremely advanced "Super-Science" that works like magic, allowing him to perform virtually anything imaginable - though he has not killed Superman and has a weakness in the form of whenever someone tricks him into speaking his name backwards he will be banished to the Fifth Dimension for a short while (though this weakness is only valid if Myx allows it (in other words he has control over his own weaknesses, showing that he is even more powerful than he lets himself be seen, likely deliberately lowering his powers for sheer "fun".)
A mischievous being from another dimension, Mr. Mxyzptlk possesses powers so advanced they can only be described as magic, which makes him a dangerous foe for Superman, as Superman's invulnerability doesn't work on magical attacks, meaning Mxyzptylk's attacks can actually hurt Superman. He's a prankster, a troublemaker and a genuine threat. He knows Superman's double identity but would never tell, fearing it would lessen the control he'd have over the Man of Steel. Besides, threatening to reveal Superman's secret is much more fun than actually doing it. If he wanted to, Mxyzptlk could kill Superman outright by simply rearranging the Man of Steel's molecules, but again, where is the fun in that? In some weird way, Mr. Mxyzptlk considers himself Superman's "pal", always showing up when least expected to throw a surprise into "the big boy scout's" life. Mxy thinks Superman actually looks forward to their running duel of wits as much as he does. Boy, is he wrong. When Mxy's not out trying to destroy Superman, he goes home to the 5th dimension where he enjoys long walks in the park, rock climbing and snuggling with his main squeeze Gsptlsnz.
Mister Mxyzptlk's powers have been confirmed for some time now to come from imagination itself, which is in turn connected to a concept in DC known as the Collective Consciousness - in short Myx has as much power as he (or those around him) imagines.
Myx is not, however, completely invincible and is below the level of beings who scale to the Over-Void, such as Mandrakk or other Monitors, he is also significantly below Perpetua.
Mxy in Superboy
Mxy in the 1960's Superman cartoon
Mxy in Batman: The Brave and the Bold
Mxy in Super Friends
Mxy in Superman: The Animated Series
Mxy in Lois& Clark: The New Adventures of Superman
Mxy as he appears in Smallville
Mxy in the Superman Returns video game
Mxyzotlk vs. Spectre
Mxyzptlk vs. Trigon
Mxyzptlk explains to Joker his power over reality
Mxyzptlk meets Impossible Man
Mxy as he appears in Justice League Action
His name's pronunciation varies from adaptation to adaptation. In the earliest adaptations it was pronounced "Mixeplick", but for Superman: The Animated Series (where Superman even initially uses the previous pronunciation), a new pronunciation was made: "Mix-yes-spit-lick".
The Impossible Man from the Marvel Comics is normally seen as Marvel's equivalent to the Mr Mxyzptlk. In the Superman story "Krisis of the Krimson Kryptonite", it was even implied the two were one and the same. Despite this easter egg however, the Impossible Man hasn't been depicted as dangerous or as powerful as Mr Mxyzptlk (the two also don't share the exact same personality), so this bit in the story isn't taken seriously by most, especially since it was just a reference/tribute to Marvel, and little else.
Similarly, after the Crisis on Infinite Earths event, Mxyzptlk took on a human disguise in his first appearance and went by the name Ben DeRoy, an anagram for Beyonder, a Marvel character whose favourite form Mxyzptlk's human guise greatly resembled.
Mister Mxyzptlk named himself such because he believed Superman would never be able to say or type it backwards (his one original weakness). Other versions have it that his name is the closest he can get to one the viewers can ever understand.
His infamous weakness is actually self-made and it only works because he allows it to work, in short he functions much like a Q from Star Trek, his weakness is as he decides, however as an Imp he must pick a weakness (this prevents Imps from destroying the multiverse with their tricks).