Cleave is a core warrior ability learned at level 44. It is the warrior's first multiple target attack. It is also used by many high-leveled bosses that mainly use physical attacks. It is a 30 Rage skill that deals weapon damage to the primary target plus bonus damage to a single additional target. If a one-handed weapon is used, the bonus damage is greater.
The actual Rage cost of Cleave can be considered to be 30 Rage + the Rage that would otherwise have been generated by your next weapon swing. In terms of efficiency, Cleave converts its initial Rage cost into bonus damage and threat.
For warriors with the Arms specialization, Cleave, like [Whirlwind], may be combined with [Sweeping Strikes] in order to generate additional hits. Cleave's secondary target is the nearest enemy other than the warrior's main target. For this reason, if the warrior and all targets are stationary, Cleave will continue to strike the same secondary target. Shifting positions slightly will often change the secondary target, however tanks and dps alike should be aware that this may cause Cleave to break nearby CC if not used carefully.
For a more in-depth discussion of the mechanics and mathematics of Cleave, refer to the Talk:Cleave/Analysis page.