Offensive stealing is when a player has crossed to the enemies spawn side of the map, and stands too close to the enemy player.
This should always result in a slay, even if "it was coming right at me."
Defensive stealing is when a player is standing on their spawn side of the map, and an enemy player is standing too close to them.
While it is generally okay to steal the rocket if it's too close to you on your side of the map, this doesn't mean you should be going out of your way to do so.
Aggressive stealing can be a combination of offensive and defensive. You can be frozen in place and still be aggressively stealing.
This is when a player has to go out of their way to steal. They can be a statue, or a rocket chaser. If they have to aim over their head, or off to the side, it's aggressive.
Passive stealing is much less common, but is still something you should be aware of.
This is when a player is in the exact path of the rocket, and simply reflects to save themselves.