Griefing is a rude act which is not actually against the ROBLOX rules, but is still not appreciated by players. Griefing is the intentional harrassment of other players in a game which can be performed in many ways. Griefing is primarily done to create pleasure from harrassment of a player/players for the griefer.
Tactics
Gears abuse
A gear abuser would kill anyone on sight except for the ones with better gears.
Killing
A griefer would usually chooses a single target, or uses very unfair methods of killing other players. This does not necessarily include VIP abuse, as some players might do it, not knowing that the others deserve a chance to defend themselves. Spawn killing is also seen as griefing, as it constitutes an action in a game called camping, in which a player would stay at an area and continually kill all enemies in the area until the camper dies.
Funny Physics
A setting which used to exist. It allowed the player's character to float in the air, and drop very slowly. The character could walk through walls, but couldn't jump, and unanchored bricks would move very strangely.