Interrupt

To interrupt an Action in Guild Wars is to prevent it from being completed. Certain Skills, which are also actions, have an activation time during which the character is preparing the skill. If a character is interrupted while preparing that action, that action can no longer be completed. A successful interrupt will cause the following result:
 * 1) The interrupted action is cancelled and has no effect.

For skills specifically, there are an additional 3 results:
 * 1) The skill's energy or adrenaline cost is forfeited.
 * 2) The skill cannot be used again until the normal recharge time for that skill passes.
 * 3) The target who was interrupted does not go through the aftercast period for the interrupted skill.

Tips on interrupts

 * To effectively interrupt, watch the Skill Monitor. which is by default under the enemy's health bar in the center of the top of the screen. When the enemy starts using a skill, the skill's activation bar appears under the bar. If it's a skill you want to interrupt (and can interrupt), do so immediately.
 * If an opponent is using a skill that takes a very long time to prepare (Resurrect, for example), it may be in your interest to wait until the last second to interrupt the player. In this case, he/she will have lost not just energy, but also the entire casting time of the spell.  Keep in mind that some skills have no recharge, so interrupting them immediately only allows the player to begin recasting the spell right away. Also be mindful that purposelely waiting that amount of time before interrupting them, that target is also regenerating energy, for the potential to activate other skills.
 * It is best to study (and perhaps play as) other professions to truly understand what is the best way to interrupt each class.
 * Interruption is not just for spells. Other actions (including attacks and attack skills and signets) can be interrupted. Attacks have an activation time (see the specific weapon's page) and take time to execute. Any such action can be interrupted, though there are more skills that interrupt spells than attacks.
 * Certain skills (such as stances and shouts) do not have an activation time. These skills cannot be interrupted.

"Easily interrupted"
Some skills are easily interrupted (including all traps). Characters activating these skills are interrupted if any attack hits, even without the use of interruption skills. All of a character's spells are easily interrupted while that character is dazed.

Other forms of skill intervention
Knockdowns, Skill Disabling and Skill Failure can also be used to prevent a successfull excecution of a skill (while Knockdowns can also prevent attacks as well). It is important to understand however, that neither of these three mechanics cause the target to be 'interrupted' (see each of the related articles for more information)

Related articles

 * Disable skills quick reference
 * Interrupt skills quick reference
 * Skill failure
 * Fail skills quick reference
 * See also: dazed and knockdown

These skills are easily interrupted:
 * Crude Swing
 * Barbed Trap
 * Dust Trap
 * Flame Trap
 * Healing Spring
 * Precision Shot
 * Spike Trap
 * Viper's Nest
 * Snare
 * Smoke Trap
 * Tripwire
 * Ethereal Light

These skills prevent interruption:
 * Mantra of Concentration prevents one interruption.
 * Mantra of Resolve prevents interruptions at the cost of energy each time.
 * Persistence of Memory makes your interrupted spells recharge instantly.
 * Glyph of Concentration prevents interruption of the next two spells cast and ignores the effects of Dazed.
 * Trapper's Focus makes traps 'not easily interruptible' for a duration.
 * Tranquil Was Tanasen is an Item Spell that prevents interruption.
 * Song of Concentration prevents interruption of the next skill used by ally in earshot.
 * Pious Concentration expends one enchantment per interrupt prevented.