Whereas autocast abilities change a unit's behaviour and cause it to move in order to cast their spell, arrow abilities are automatically used when the unit attacks, as long as they are toggled and the unit has the required mana to cast the ability.
Only one arrow and one autocast ability can be active at the same time.
Arrow abilities are more attack modifiers than actual spells, as they do not deal damage by themself, but rather increase the damage of the attack. This means, that the damage is physical and has the same attack type as the normal attack. As a result the damage from arrow abilities is reduced by armor, but not by Runed Bracers or Elune's Grace.