Gives X additional armor to party members within 30 yards. Players may only have one Aura on them per Paladin at any one time.
"By safeguarding those around him, a paladin aids in his own defense." —Uther the Lightbringer[1]
Devotion Aura is a paladin ability that creates an aura around the caster, increasing his Armor by a certain amount (depending on the aura's level). It has the same effect on all nearby party/raid members.
Whereas pre-WotLK Retribution Aura was the aura of choice for Paladin Tanks, with the massively increased threat generation post 3.0 Devotion Aura is generally preferred for it's added mitigation and bonus to healing from Improved Devotion Aura.