Ganking is when a big group of players team up on one lone player, usually by surprise. Level differential is not necessary to be called ganking. In many games (mostly MMOs), this is a form of griefing, but in other games it is a legitimate and encouraged strategy.
For example, in DotA, ganking is an integral part to team victory. One player may roam around the map helping to ambush and team up on enemies who get caught out of position.
A metagame for any game is the way that the game is played. Knowing the metagame is not just knowing the mechanics of the game, but knowing what to expect your opponent to do. It is the relative frequency of each possible tactic, maneuver, build, etc.
One of the easiest to understand examples of this is with collectible card games like Magic: The Gathering and Hearthstone: Heroes of Warcraft. Knowing the metagame in a collectible card game is knowing what styles of decks you might come up against. For example in Magic if your opponent plays an Island on turn one, are they attempting to lock you down, draw you out, make evasive creatures, or any variety of things blue Magic players can do? You don't know if you don't know the metagame.
In StarCraft, for a more localized example, Protoss should wall off against Zerg, but not against Protoss or Terran. Zerg might 6-pool, and if you don't wall off then your opponent is probably going to beat you. As Protoss continues to wall off and 6-pools continue to be ineffective, Zerg players might stop using that strategy. As Zerg players stop 6-pooling, Protoss players might not wall off as often, making the 6-pool effective again. You have to know the current mindset of other players to make the optimal play. That is the metagame.
The metagame is always evolving for any game, so you have to keep playing or reading about your game to keep up with the current metagame.
Best Answer
Griefing is the deliberate, indiscriminate and repeated harassment of other players - with generally little actual benefit to one's own character.
It is essentially a "play style" where the player derives satisfaction from the discomfort he causes others instead from the accomplishments provided by the game itself.
This stands in contrast to unintentional bad behavior resulting from inexperience and also in contrast to harassment of specific players as can be part of a personal feud.
Some examples:
Like Mechko said: Bullying in the gaming world...