Activated abilities are written in the form "[Cost]: [Effect]." Some keyword abilities, such as equip, are activated abilities and will have colons in their reminder texts. Loyalty abilities of planeswalkers are activated abilities.

Triggered abilities use the word "when," "whenever," or "at." They're often written as "[Trigger condition], [effect]." Some keyword abilities, such as afterlife, are triggered abilities and will have "when," "whenever," or "at" in their reminder text.

An activated mana ability is one that adds mana as it resolves, doesn't have a target, and isn't a loyalty ability. A triggered mana ability is one that adds mana and triggers on an activated mana ability.

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.

If you counter a delayed triggered ability that triggered 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.