The Energy Bar

Energy powers most skills and spells, so maintaining this resource is critical in battle. The Energy Bar displays the amount of Energy you have with which to use skills. Each skill has an Energy cost associated with it, and using a skill will deplete that cost from your Energy Bar (note that sometimes the Energy cost is zero as is the case with most Signets and adrenal skills that draw on a different resource). Your maximum Energy is determined by your profession's armor, though some skills and conditions temporarily increase or decrease your maximum Energy. Some magical items provide a permanent or temporary increase, and Elementalists have the unique Energy Storage attribute which can increase their maximum Energy. Different professions have different rates of Energy regeneration, with Warriors at the low end and casters at the higher end.

When you are under a Hex or some other negative Condition, your Health and/or Energy may degenerate, as indicated by the arrows in the Health or Energy Bar. Left arrows [<<<] indicate degeneration, while right arrows [>>>] indicate regeneration. More arrows mean faster Health and Energy regeneration or degeneration. Regeneration and degeneration can accelerate rapidly, so keep an eye on them. http://www.guildwars.com/gameplay/combat/stayingalive/