Powergaming/Metagaming are both basically the same thing - Using information or knowledge that your character should not have to meet a situation. For example, if I see something on one character, I shouldnt use that knowledge on another character of mine who was not present at the time. Each character is it's own entity and not connected to the rest unless some form of communication goes on between them.
It doesnt even have to be character related. There are secrets in this game that should only be known by certain individuals, but due to forum access, wiki access, and other sources outside of the game, people figure out these secrets and use them against the factions involved.
Metagaming/Powergaming is also used to describe a situation where one player roleplays for everyone else present in a forceful manner, against the will of the other players involved. For example, a Gunboat faces off against a Battleship, and the gunbaot pilot roleplays that his crew boards the battleship and kills the battleship captain. The battleship player has no way to defend against this, and did not agree to the action in the first place, so it's considered powergaming. Another similar situation is RP'ing the destruction of a defeated player's escape pod to 'kill' the player - nobody is going to willingly delete their character because someone says their escape pod was shot, so it's considered powergaming.
Powertrading is generally the use of a 5k Transport to do a route that isnt considered in-character for the character's faction or affiliation. Powertrading is done solely as a means to make as much money as possible as quickly as possible, with no regard for roleplay at all.