Action

Action describes an effect that does something on resolution (as opposed to a Cost which does something on activation, or a Condition which must be met before the effect can be activated)

Types of Action
Trigger - A trigger action happens when something else happens (for example, the End Phase was entered, or a Spell was activated). This is Spell Speed 1 by default, and will activate at the end of a chain. If you put the Quick keyword with it, it will be Spell Speed 2 and potentially chain itself to the thing that happened.

Ignition - An ignition action does not require any other action to be activated, the user chooses to activate it when they please. It is always Optional. By default, it is Spell Speed 1 and can only be activated in the player's Main Phases. If you put the Quick keyword with it, it will be Spell Speed 2 and can activate during either player's turn, during any phase. To restrict the phases it can be activated in, add an Ignition Condition.

Continuous - A continuous action does not start a chain or do something a set number of times; it is always "on". It is always Mandatory (not Optional). It is only active when the card is face-up on the field by default, but can include the Graveyard, Banish Zone, etc. with a Continuous Condition.

Inherent - An Inherent action is similar to an Ignition action, but cannot and does not start a chain (like Cyber Dragon's Special Summon). By default it is only usable while in the hand, but can include the Graveyard, Banish Zone, etc. with an Inherent Condition. They usually mean Summoning Conditions, and are often used with REDEFINE. Unlike Ignition actions, they can be Optional or Mandatory.

Universal - A Universal action is similar to a Continuous action, but it is processed at the start of the duel, and applies wherever the card is (like Harpie Lady 1, 2 or 3)

Modifiers
Optional - All Trigger and Inherent actions are not optional by default. This keyword prompts the user if they want to use the effect or not.

Quick - This keyword changes Trigger and Ignition actions to Spell Speed 2. Ignition effects will also, by default, be usable during any phase of any turn.

STActivation - Used for Spells and Traps, this keyword means this action is coupled with the actual Spell/Trap activation (for Continuous Spells and Traps, the point where you place it on the field face-up). For example, some spells and traps have an effect that activates right when you play the card (like Call of the Haunted). Also see its use in Conditions.

NoNegate - The effect cannot be negated by any card effect, except by other effects in the same card, or those with Ignore

Lingering - The effect operates independently of the card (For example - Final Countdown. The actual card goes to the graveyard, but the effect continues on. For a Normal Spell or Trap, as soon as it encounters a lingering effect it will be sent to the Graveyard)

Ignore - Ignores conditions.