Adrenaline

Certain Warrior, Paragon, and a few PvE Skills are fueled by adrenaline rather than energy. It is gradually built up each time the character hits or is struck in combat, each such blow increasing the character's adrenaline skill's levels. There are skills that grant adrenaline, take it way, and increase or decrease the rate in which it is built up.

Adrenaline Points
Adrenal is measured in Adrenaline points. Skill description's reference to cost in strikes is a simplification. A strike is 20-25 points depending on the skill. As the table below shows, many Warrior skills and all Paragon skills use a 25:1 ratio.

See this table for number of adrenaline points needed for skills.

1 4 "strikes" on Cleave does not equal 4 "strikes" on Sever Artery. The "Points needed" column increases linearly.

Gaining adrenaline
Every adrenaline skill stores its own pool of adrenaline points displayed by rising flames from 0 until fully charged. Gaining adrenaline means each non-disabled non-recharging adrenal skill in your bar gains that many points.
 * For every successful hit (or 'strike') you make, you gain 25 adrenaline points.
 * For every 1% of health lost from your maximum, you gain 1 point of adrenaline. Runes count against the maximum health. Gain is calculated on a per-hit basis. Health lost includes attacks and spell damage but excludes health degeneration and life stealing skills. Commonly, 4 strike skills are charged in about 3 hits time, because you take some damage at the same time, assuming that the opponent is attacking you.
 * Taking less than 1% damage (including zero damage) causes you to gain zero adrenaline (that matters for 'Lose All Adrenaline' below).
 * When fully charged, Adrenal skills gain no more adrenaline; a skill cannot be 'overcharged'.

Using adrenaline
You can only use adrenal skills that are fully charged with adrenaline. They cost 0 Energy. When you use an adrenal skill, that skill's adrenaline pool is emptied, and all other adrenal skills in your bar lose 25 points.

Losing adrenaline

 * When you use a fully charged Adrenal skill, all of that skill's adrenaline is lost (i.e., spent).
 * Some skills cause you to 'Lose all adrenaline' which means every adrenal skill in your bar loses its full load of adrenaline. For attack skills with this feature, the effect comes after the 25 points each skill gains if the attack hits, so all pools will be at 0. These skills are: Hammer Bash, Heavy Blow, Wild Blow, Final Thrust, "On Your Knees!", Shove,  Burst of Aggression, and Decapitate.
 * You 'Lose All Adrenaline' after you have not gained any adrenaline for 20 seconds, and when you die. Your uncharged adrenal skills blink a couple of seconds before this happens.
 * You will not lose adrenaline even if you take 0 damage from an attack.
 * Skills like Rage of the Ntouka let you gain and maintain adrenaline outside of combat, usually letting you start the next battle with all your adrenal skills fully charged.
 * Any adrenal skill that is reset, disabled, or recharging will lose all adrenaline and will not gain any new adrenaline until it has recharged. Diversion and Shield Bash can be used to disable individual skills. Blackout can disable multiple skills of an opponent. Bestial Fury, Shove, Marauder's Shot, and Tiger's Fury can disable your own skills.
 * Sympathetic Visage and Ancestor's Visage causes a loss of all adrenaline for all adjacent foes when the enchanted ally is struck by a melee attack.

Related articles

 * Gain adrenaline skills quick reference
 * Adrenaline skills quick reference
 * Adrenaline denial

Reference
Original GW Guru thread.