An attack is an action by a player or monster intended to cause damage. Player attacks are all tied to the activation of a skill, and there is no default attack. Right-clicking a skill will select it as auto-attack, activating on recharge until cancelled.
The game does not differentiate between attack types like in the original Guild Wars. Spells and other skills which don't directly involve attacking with your weapon are still considered attacks. The player attribute power directly increases damage done by all attacks.
The warrior mechanic of adrenaline gain is tied to the player attacking. Monster attacks are not tied to the player skill system and often have special attack abilities which unlock as more players get involved in the fight.
The boon "might" increases attack.