Interrupt

To interrupt an action in the game is to prevent it from being completed. Every skill in the game has an activation time during which the character is preparing the spell. If a character is interrupted during skill activation, the skill is interrupted. This has three results:
 * 1) The skill will not be executed obviously.
 * 2) The cost of the spell (if it costs energy or adrenaline) is lost to the character.
 * 3) The character must wait until the recharge time for that skill passes by before using it again.

There are many skills in the game that allow a player to interrupt a character's actions:
 * Elementalist: Lightning Javelin
 * Mesmer: Leech Signet, Clumsiness (interrupts attacks only), Power Spike, Power Leak and Power Drain which interrupt spells only.
 * Monk: None.
 * Necromancer: Spinal Shivers
 * Ranger: Punishing Shot, Concussion Shot (spells only), Savage Shot, Choking Gas (spells only) and Distracting Shot.
 * Warrior: Distracting Blow, Disrupting Chop

In addition, any skill that knocks down an enemy (like Hammer Bash or Bane Signet) will interrupt their current action.

Tip: To effectively use interrupting attacks, keep an eye on the enemy's bar in the center of the top of the screen. When the enemy starts using a skill, the icon of that skill will appear under the bar. If it's a skill you want to interrupt (and can interrupt), go for it!

Easily interrupted
All nature Rituals are Easily interrupted while casting, whatever that exactly means.