Counter target spell, activated ability, or triggered ability. (Mana abilities can't be targeted.)
2/9/2017 Activated abilities are written in the form “Cost: Effect.” Some keyword abilities, such as equip and crew, are activated abilities and will have colons in their reminder texts.
2/9/2017 Triggered abilities use the word “when,” “whenever,” or “at.” They’re often written as “[Trigger condition], [effect].” Some keyword abilities, such as prowess and fabricate, are triggered abilities and will have “when,” “whenever,” or “at” in their reminder text.
2/9/2017 If you counter a delayed triggered ability that triggers at the beginning of the “next” occurrence of a specified step or phase, that ability won’t trigger again the following time that phase or step occurs.
2/9/2017 An activated mana ability is one that adds mana to a player’s mana pool as it resolves, doesn’t have a target, and isn’t a loyalty ability. A triggered mana ability is one that adds mana to a player’s mana pool and triggers on an activated mana ability.
2/9/2017 Abilities that create replacement effects, such as a permanent entering the battlefield tapped or with counters on it, can’t be targeted. Abilities that apply “as [this creature] enters the battlefield” are also replacement effects and can’t be targeted.
1/19/2018 There are many important moments in the story, but the most crucial—called “story spotlights”—are shown on cards. These cards have the Planeswalker symbol in their text box; this symbol has no effect on gameplay. You can read more about these events in the official Magic fiction at http://www.mtgstory.com.