I know what you're all thinking: "What's this about Greeks and jars and shit? None of this was in Avatar." Well, you're about to get schooled in Greek mythology.
Just The Facts
1. Pandora's Box is a Greek myth that tells the story of how a woman called Pandora was inadvertently responsible for Mankind's suffering.
2. Despite what you may have learned from God of War, it does not enable you to have a duel with Ares by making you giant.
3. The Greek word 'pithos' actually means 'jar' but was mistranslated as 'box'. So the story should really be called 'Pandora's Jar'.
4. Of course, that doesn't leave as many opportunities for jokes about female genitals.
5. E.g. "Man, I opened Pandora's Box so hard last night."
The Story Itself
So, in this place called Greece (you may have heard of it), there was a dick called Zeus. It was very unfortunate that Zeus was a dick, because he was, you know, King of the Gods. Zeus was such a big dick, that he didn't want mankind to have fire. It could better their lives by warding off dangerous beasts and helping them not freeze to death. And that's terrible.
But there was a Titan, Prometheus, who was much less of a dick. He thought; "It's kind of unethical for Zeus to hog all this fire. I know! I'll be a good egg and give it to the humans to improve their overall quality of life." So Prometheus stole the secret of fire and gave it to mankind. And so, in a rare moment of compassion, Zeus realised what a douchebag he was, and resolved to only do good deeds from now on, to set a better example for humanity to follow...
Nah, I'm just kidding. He chained Prometheus to a rock and had an eagle tear his liver out every day for all eternity.
But that wasn't enough for Zeus (whose name, interestingly enough, was a colloquial term in ancient Greece for "complete cockbag"), who decided that mankind needed to be punished for Prometheus' act of kindness (no it doesn't make any sense. Just roll with it.). So he got Hephaestus to make the first woman, Pandora. Which is a bad thing, apparently. All the Gods helped in her creation. Of particular note were Hermes' contributions: "A shameful mind and a deceitful nature" and he also put "lies and crafty words" in her mouth.
I can hear the Feminist Extremists knocking on my door already.
So, moving on from these horrific implications, the Gods gave Pandora to Prometheus' brother Epimetheus as a present, probably sniggering and making up excuses about jokes they'd been told earlier all the while. When the two got married, Zeus gave Pandora a jar for a wedding present which he told her never to open. It isn't mentioned whether or not he placed a giant neon sign on the jar which said "PLEASE OPEN ME", so we'll have to assume that he did.
So one day, when Epimetheus was nowhere to be found, Pandora thought "Hey, Zeus gave me that jar. And then he said not to open it. OMG LETS OPUN IT." , or something to that effect. So she did, in a move that probably gave Jedi all around the galaxy a massive headache.
So instead of jewels or dresses or whatever Pandora thought would be inside the jar, it turns out it contained bad things. That's not comedic frankness, it actually contained what can best be described as bad things. Greed, poverty, death, misery, the Star Wars Holiday Special and Justin Bieber.
In what must have been the most intesnse "Ohshitohshitoshitoshitoshit" moments ever, Pandora slammed the lid back on the jar. But there was still a little voice in the jar still begging to be set free. Pandora reasoned that nothing could be worse than what she'd just let out, and so she removed the lid, and a little thing called Hope (you may have heard of it) fluttered out.
Shamelessly copied from Cracked.com