Resources are what players have to use to fight the enemies they encounter.
Energy is used by Rogues and Druids in Cat Form. Players start with 100 Energy, and the amount goes down when abilities are used. Energy constantly recharges, and is affected by Haste Rating. Energy has a base regeneration rate of 1 Energy per decisecond. (i.e. 1 Energy per tenth of a second). Energy regeneration rate is not affected by combat. It has a ceiling of 100.
Focus is used only by Hunters and their Pets. It is very similar to Energy, however it has a slower base regeneration rate than Energy, at 6 Focus per second. Focus regeneration rate is not affected by being in combat. It has a ceiling of 100.
Mana is the primary resource of Caster classes. Druids (When not in a Feral form), Mages, Paladins, Priests, and Warlocks all use Mana as their main resource. Mana is a finite resource, though spells and items do exist that replenish some or all Mana during combat, in addition to the combat regeneration rate of mana.
Rage is used only by Warriors and Druids in Bear Form. Rage starts out at 0, and is generated by attacking and being attacked. Rage is then "spent" on using abilities. Rage has a ceiling of 100, and diminishes when not in combat.
Runes are used exclusively by Death Knights. Death Knights have 2 Blood Runes, 2 Unholy Runes and 2 Frost Runes at their disposal, although these Runes can be turned into Death Runes via abilities; Death Runes are treated as any other Rune. Runes are "spent" to use abilities, and once spent, they regenerate, becoming available to spend again after a short delay. In essence, each Rune acts similar to an Energy bar, that can only be spent once it is full.
For a fuller explanation, see Death Knight Rune System.
Runic Power is used exclusively by Death Knights. Runic Power starts at 0, and is only generated by attacking enemies with special attacks. It is spent on special attacks to generate time for Runes to recharge for other abilities. It has a ceiling of 100, but can be extended to 130 with the correct Talents. Runic Power diminishes when not in combat.
Steam is used on some vehicles. It usually shares the same Regeneration/Cost mechanic as Energy, although there are some instances where manual replenishment may be required.