Powergaming is when you force consequences without consent on someone else's character. For example, claiming the opposing character died during a fight when you shot the ship down. The only person deciding whether a character dies or survives is the owner of a character. You have no say over other people's characters, as that would be powergaming.
Metagaming is, like BretPriv wrote, having your character have access to information your character can't have. Characters need to earn their knowledge inRP, not by gaining it from the creator of the character. Examples for metagaming are your character knowing things that another of your characters witnessed (for example, you have a trader in Liberty and see a smuggler moving towards New York, log off and log an LPI character to intercept the smuggler) or on forum for example accessing comms that aren't directed at you or taking information from other people's story threads. Matter of factly, having your inRP character use the player list is also considered metagaming.