Every character has a resource that is needed to perform most abilities. For most classes, this is mana, and some players will simply refer to all these resources as mana. However, rogues use energy and warriors build rage which use slightly different mechanics. The only thing energy and rage have in common with mana is that they are used to perform abilities.
All classes except rogues and warriors use mana. These classes, altogether, are often called casters. However, hunters are usually excluded from this list since their abilities are generally not spells. 
A mana bar is colored in blue, and the amount of mana a character has grows significantly as they level, just like their health. The total amount of mana a player can have is called their mana pool. 1 point of intellect will increase the mana pool by 15. When a mana user casts a spell, they will use up some amount of mana, and if it is not available they cannot cast the spell. Because higher ranks of an ability have a stronger effect, they also consume more mana. Using mana wisely is important since it is difficult to regain, at least while in combat.
Mana Regeneration allows a player to recover some mana as time goes on. A caster's natural mana regeneration is tied to their spirit. However, any time an ability is used that consumes mana, it triggers the "5 second rule" that prevents any regeneration for the next 5 seconds. This means that if a player is casting spell after spell without pause, they will probably not regenerate mana during this time period.
The attribute "mana per 5 seconds" (or MP5,) will continue to operate. Although MP5 tends not to be quite as strong, it is considered vital by many casters at higher levels.
The other common method of regaining mana is through drinking, however, this can only be done out of combat.
Rogues and druids in catform use a different system. They usually start with 100 energy, . When they use an ability, it takes a chunk out of the energy bar, and every 2 seconds they get 20 points back. The energy cost of their abilities does not increase with rank. This means that while a rogue cannot constantly pour out attacks one after another like a mana user can, they do not have to worry about running out of mana either. They steadily generate energy to perform abilities till the end of a battle, no matter the length.
Many rogues form complex systems of how they open their battles to use their initial 100 points of energy in the most optimal manner possible. Many talents reduce energy costs or increase energy generation to make them more effective. See Adrenaline Rush.
Druids in cat form behave like a rogue and also use Energy.
Hunter pets have an attribute called Focus that behaves identically to Energy.
A Death Knight's ghoul, summoned with the Raise Dead ability also uses Energy.
The warrior's rage behaves very differently. While it is still spent on the use of abilities, the means of gaining rage are unique.
By default, energy and mana tend to fill up when not being used. Rage, on the other hand, gradually depletes when it isn't being used, and warriors often begin a battle with no rage whatsoever. However, every time a warrior attacks or is attacked, they gain a few points of rage, to a maximum of 100.
Warriors also have a few abilities that add to rage and certain abilities that can be used without any. Charge will add up to 15 rage when used to begin a battle, and Blood Rage also grants the warrior several points of rage, and can be used out of combat. Also, because rage tends to build up more slowly than energy, the cost of warrior abilities is generally much lower.
Unused rage will remain in the warrior's pool until it is used or combat ends. At the end of combat, rage gradually dwindles.
One additional side effect of rage is that it is nearly or entirely lost when changing stances. While most battles will be fought in a particular stance, this makes it prohibitive to, for example, switch from defensive stance to berserker stance to intercept a target that has run away, and then switch back to defensive.
A large number of warrior talents affect the generation or use of rage in some way.
Druids in bear form also use Rage.
Unique to the Death Knight Class, Runes and Runic Power are heavily linked.
Playing a Death Knight is all about managing different resources.
Death Knights have 2 Blood Runes, 2 Frost Runes, 2 Unholy Runes and a Runic Power bar that goes from 0 to a 100. Many talents can also turn your runes into Death Runes, which count as any of the 3 types.
Abilities either cost Runes, or Runic Power, as indicated by their tooltip.
Some abilities may cost more than 1 rune to activate. When you use a rune, it needs to recharge for 10 seconds. Some talents can cut down the time it takes for rune to recharge. Every time a runic ability is used, you gain 10 Runic Power per runes expanded.
Runic Power starts at 0, and quickly goes down when outside of combat, much like Rage. Abilities that cost Runic Power either cost a fixed amount (Such as Death Coil which costs 50 Runic Power) or cost a certain amount to activate, then drain Runic Power every second until their duration ends or the Death Knight's runic power reaches 0 (such as Summon Gargoyle, which costs 40 Runic Power and then drain 8 per seconds).
Focus has now replaced mana as the hunters source of using spells. You start with 100 focus. As you use skills your focus is used up but is constantly regenerating. There are many different ways of gaining more focus such as using the instant spell Frevor. You can also regenerate focus by casting Steady Shot or Cobra Shot which increase give back 9 focus after successfully being cast.
Hunter's pets have been using focus for a very long time and now hunters get to behave like their pets.
Focus is very similar to rogues and druids "energy" source.
NPCs will only ever show mana bars. They do not visibly generate energy or rage. Instead, their abilities simply have cooldowns, and the AI for the game chooses when to use each ability.
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