glitches, such as, you get money from something repeatedly (it's getting generated somehow)
Basically anything that benefits you that you continue to use and isn't supposed to happen.
Block jumping is when you're in a region you don't have access to and you climb two high blocks by placing a block for a split second and using it to boost yourself. 100% illegal.
A dark room is usually underground, basically a player-made area where lots of mobs spawn.
In your example, I'm not 100% sure but I think it's legal. Killing them that way will not give you special rewards, though.