An activated ability is an ability that can be activated by a player by paying a cost. The cost can be almost anything, such as sacrificing a permanent, paying mana, paying life, or tapping the permanent with the ability (). Activated abilities are usually found on permanents, but can sometimes be used on cards in other zones, such as the graveyard or a player's hand.
Only an object's controller (or owner, if it has no controller) can activate its activated abilities unless the ability says otherwise. Also, you can usually activate an activated ability any time you have priority, unless it says otherwise. Except for those that are mana abilities, activated abilities use the stack.
A player cannot activate an activated ability of a creature that includes the tap symbol () or the untap symbol () in its cost unless he or she has controlled the creature continuously since the start of his or her most recent turn. This condition is informally known as "summoning sickness".
Activated abilities cannot be countered by spells or abilities that counter spells, because they aren't spells. However, there are cards, such as Stifle, Squelch and Voidslime, that can be used to counter these abilities.
 Comprehensive Rules
From the Comprehensive Rules (Return to Ravnica)
- 602. Activating Activated Abilities
- 602.1. Activated abilities have a cost and an effect. They are written as "[Cost]: [Effect.] [Activation instructions (if any).]"
- 602.1a The activation cost is everything before the colon (:). An ability's activation cost must be paid by the player who is activating it.
Example: The activation cost of an ability that reads ", : You gain 1 life" is two mana of any type plus tapping the permanent that has the ability.
- 602.1b Some text after the colon of an activated ability states instructions that must be followed while activating that ability. Such text may state which players can activate that ability, may restrict when a player can activate the ability, or may define some aspect of the activation cost. This text is not part of the ability's effect. It functions at all times. If an activated ability has any activation instructions, they appear last, after the ability's effect.
- 602.1c An activated ability is the only kind of ability that can be activated. If an object or rule refers to activating an ability without specifying what kind, it must be referring to an activated ability.
- 602.1d Previously, the action of using an activated ability was referred to on cards as "playing" that ability. Cards that were printed with that text have received errata in the Oracle card reference so they now refer to "activating" that ability.
- 602.2. To activate an ability is to put it onto the stack and pay its costs, so that it will eventually resolve and have its effect. Only an object's controller (or its owner, if it doesn't have a controller) can activate its activated ability unless the object specifically says otherwise. Activating an ability follows the steps listed below, in order. If, at any point during the activation of an ability, a player is unable to comply with any of those steps, the activation is illegal; the game returns to the moment before that ability started to be activated (see rule 717, "Handling Illegal Actions"). Announcements and payments can't be altered after they've been made.
- 602.2a The player announces that he or she is activating the ability. If an activated ability is being activated from a hidden zone, the card that has that ability is revealed. That ability is created on the stack as an object that's not a card. It becomes the topmost object on the stack. It has the text of the ability that created it, and no other characteristics. Its controller is the player who activated the ability. The ability remains on the stack until it's countered, it resolves, or an effect moves it elsewhere.
- 602.2b The remainder of the process for activating an ability is identical to the process for casting a spell listed in rules 601.2b–h. Those rules apply to activating an ability just as they apply to casting a spell. An activated ability's analog to a spell's mana cost (as referenced in rule 601.2e) is its activation cost.
- 602.3. Some abilities specify that one of their controller's opponents does something the controller would normally do while it's being activated, such as choose a mode or choose targets. In these cases, the opponent does so when the ability's controller normally would do so.
- 602.3a If there is more than one opponent who could make such a choice, the ability's controller decides which of those opponents will make the choice.
- 602.3b If the ability instructs its controller and another player to do something at the same time as the ability is being activated, the ability's controller goes first, then the other player. This is an exception to rule 101.4.
- 602.4. Activating an ability that alters costs won't affect spells and abilities that are already on the stack.
- 602.5. A player can't begin to activate an ability that's prohibited from being activated.
- 602.5a A creature's activated ability with the tap symbol () or the untap symbol () in its activation cost can't be activated unless the creature has been under its controller's control since the start of his or her most recent turn. Ignore this rule for creatures with haste (see rule 702.10).
- 602.5b If an activated ability has a restriction on its use (for example, "Activate this ability only once each turn"), the restriction continues to apply to that object even if its controller changes.
- 602.5c If an object acquires an activated ability with a restriction on its use from another object, that restriction applies only to that ability as acquired from that object. It doesn't apply to other identically worded abilities.
- 602.5d Activated abilities that read "Activate this ability only any time you could cast a sorcery" mean the player must follow the timing rules for casting a sorcery spell, though the ability isn't actually a sorcery. The player doesn't actually need to have a sorcery card that he or she could cast.
- 602.5e Activated abilities that read "Activate this ability only any time you could cast an instant" mean the player must follow the timing rules for casting an instant spell, though the ability isn't actually an instant. The player doesn't actually need to have an instant card that he or she could cast.
Below are examples of different activated abilities.
Lore Broker's activated ability can be activated any time the controlling player could cast an instant, which means he or she must have priority to activate it. The cost includes the tap symbol , which means that because Lore Broker is a creature, the ability cannot be played if the controller has not controlled it from the beginning of his or her turn. (This is informally known as "summoning Sickness".)
Nezumi Bone-Reader's activated ability's cost does not include the symbol, which means it can be activated even if it has summoning sickness. However, it can only be activated during the controller's main phase when he or she has priority and the stack is empty.
Squallmonger's ability can be activated by other players than the controller, because the card explicitly says so. The player activating the ability must still have priority in order to do so, however.
Ghost-Lit Raider has two activated abilities. The card doesn't explicitly say when you can activate the two different abilities, but it is inferred by the cost.
The first requires the controller to tap it, and thus requires the card to be on the battlefield, as only permanents can be tapped. The second (which has the ability word Channel) requires the controller to discard it from his or her hand. This implies that it can only be activated when it is in its controller's hand.