Aggressive mobs will have a red name tag above them.  Aggressive mobs attack any passive mobs  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 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.) 
Passive mobs will have a green name tag above them.  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!)