Aggressive
Great White Shark attacking a StingRay
Aggressive mobs will have a red name tag above them. [1] Aggressive mobs attack any passive mobs [2] and attack the player if they are in range.Aggressive mobs will mostly appear in nighttime or dark places like caves. They will chase anything passive,neutral and the player.
Neutral
Neutral mobs will have a yellow name tag above them. They will not attack anyone unless someone provoked them(attacked them). They will automatically chase down and attack hostile mobs on range.(No matter if the hostile mobs attacked them or not,neutral mobs will always attack them.) [3]
Passive
Passive mobs will have a green name tag above them. [4] Passive mobs will not attack you no matter what you do.Passive mobs almost always spawn during the day and spawn rarely in night.(unless there are spawners placed duh!) [5]
Trivia
Before 10.4.1 Update All Mobs Gets damaged on fall when Mobs fall does not crashed
In 10.4.1 Update Mobs gets Damage on Fall
Drop with Monsters when Player deal 25% damages
During 6.0.1 Update Mobs can attack each other
During 6.0.4 Versions Neutral Mob can Attack Aragog