Instantly conjures a magical barrier around the caster which makes them basically invincible, unless you're a Priest with Mass Dispel. Even the Warlock's Chaos Bolt, which is supposed to punch through just about anything, is rendered useless against this technique. Lasts twelve seconds.
Infamous for its use as a last-resort maneuver. That is, when a Paladin is getting his sorry ass handed to him, he'll bubble up, utilizing his invulnerable state to either heal or run (often through the aid of a Hearthstone, called "bubble-hearthing").
When a Paladin bubbles and tries to heal, you mind as well bandage yourself, keep your distance, and evade if he comes for you, since there's nothing you can do to him.
Good news is, the cooldown is pretty long, so they'll only be able to use it once during your combat if they've got it ready. Plus, healing drains mana, which a paladin needs to execute any of his powerful attacks. So the longer you can survive, the better off you'll be.
Most non-Paladins frown upon bubbling, especially the ones that don't know what to do when it happens.
See also Bubbleboy.
Paladin: Oh, hell, oh, hell! HP low! *Divine Shield, initiates Greater Heal.*
Rogue: FFS! *Bandage, Vanish.*