Bubble

Experience Bar
The experience bar on the upper part of the interface bar is separated into segments that each represent 5% of the total XP needed to advance to the next level. Sometimes a segment is called a bubble. The number of bubbles is always twenty, but the amount of experience to advance to the next level changes as you go up in levels, so the amount of XP represented by each bubble also increases.

Class Abilities
Bubble can also refer to numerous class abilities that create a bubble-like animation around a character. Most commonly, they refer to the Paladin spells Divine Protection, Divine Shield, and Blessing of Protection. It can also sometimes refer to the Priest's Power Word: Shield, Mage's Mana Shield, Ice Barrier, and Voidwalker's Sacrifice. Mostly "Bubble-spells" are seen as lame by other players because they cannot do anything (in case of a paladin) or virtually less (to other classes). Paladins in particular are harassed because of their bubbles, being dubbed "bubble boys" or the like. This may be out of partial jealousy, a paladin's bubble is undeniably the strongest of the game's bubble spells, and a paladin can completely turn the tide of a battle with his bubble because he can also heal (Paladin bubbles only reduce OFFENSIVE effectiveness). And don't forget the classic Bubble Hearth, the Paladins signature move.

Chat Bubble
This is the bubble of text that shows above your character when you or others speak. It can be turned on an off via the options screen.