Energy is used to power many of the special attacks available to Rogues and Druids in Cat Form. It is represented by the yellow bar near your portrait in the upper left of the screen. Unlike your health, energy does not increase as you gain levels but is always measured on a scale of 0-100. Whenever you use a special attack or ability that requires energy (this will be noted in the tooltip of the ability), your energy is reduced by the appropriate amount. Energy regenerates quickly, so even if you use all of yours up, it will come back in a few seconds and be ready to use again.