The Aid spell gives its target creature the benefits of a Bless spell, plus 1d8 additional hit points for the duration of the spell. If additional hit points will put target above his maximum hit points they will become temporary hit points.
The recipient of this spell gains the benefit of a Bless spell (+1 to attack rolls and Saving Throws) and a special bonus of 1d8 additional Hit Points for the duration of the spell. The aid spell enables the recipient to actually have more Hit Points than his full normal total. The bonus Hit Points are lost first when the recipient takes damage and they cannot be regained by curative magic.
Example: A 1st-level fighter has 8 Hit Points, suffers 2 points of damage (8 - 2 = 6), and then receives an Aid spell that gives 5 additional Hit Points. The Fighter now has 11 Hit Points, 3 of which are temporary. If he is then hit for 7 points of damage, 4 normal Hit Points and all 3 temporary Hit Points are lost. He then receives a Cure Light Wounds spell that heals 4 points of damage, restoring him to his original 8 Hit Points.