Defensive Auras is the Second of the Paladin's Skill Trees, and encompasses a number of skills, each with its own specific protective effect. As the name suggests, the skills contained therein are primarily defensive, though some have a healing/regenerating effect, such as Meditation or Prayer.
For the Paladin, the Defensive Auras are of great importance in multiplayer games because of their ability to protect/affect other party members. As with the Offensive Auras, the Defensive Auras are great in single-player, but really shine when they can be applied to a team.
A Paladin cannot use more than one Aura at the same time. Different Auras can be stacked by multiple Paladins.
The Act II Desert Mercenary also uses Auras which may stack with a Paladin.