these are terms used by people to rant about "others" and call them names. - ( well, except metagaming )
but for the sake of definition:
powergaming:
playing to have your will/way or to win on every level. - so if you cannot win RPwise, you ll try ooc.. you extend the powers of your character beyond whats possible for this character in terms or roleplay - often using ways only the player has access to.
metagaming:
using knowledge the player has, but not the character. - that includes knowledge about close communications, diplomacies, secret deals etc. and make it so that they affect the characters roleplay or even activly interfering with it. - it includes pre-emptive stikes based on knowlede one does not have.
powertrading:
basicly the trading of goods for personal gain only. - regardless of how much a route is in RP or not. - in the recent past also applied to traders doing 100% RP runs though - and applied by people who think that others shouldn t earn as much as they do.
all three are for 90% hypocracy - and for 10% actual problems.