An activated ability has a "Cost: Effect" format. Look for the colon. A triggered ability starts with "when", "whenever", or "at".

Turn-based actions and special actions like the normal card draw, combat damage, or turning a face-down creature face up can't be targeted.

It can target delayed triggered abilities. For example, a card that says "Whenever a player cycles a card, you may exile target creature. If you do, return that creature to the battlefield at the beginning of the end step." triggers when a player cycles a card and creates a delayed trigger that happens at the beginning of the end step. You can choose to target the "at the beginning of the end step" trigger when it is placed on the stack at end of turn.

Stifle : règles, fonctionnement et interactions (MTG)

Stifle is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Counter target activated or triggered ability. (Mana abilities can't be targeted.)

How it works : Stifle

Original English text — Wizards of the Coast / Scryfall

#1October 4, 2004· Wizards of the Coast
An activated ability has a "Cost: Effect" format. Look for the colon. A triggered ability starts with "when", "whenever", or "at".
No other card is named in this ruling.
#2October 4, 2004· Wizards of the Coast
Turn-based actions and special actions like the normal card draw, combat damage, or turning a face-down creature face up can't be targeted.
No other card is named in this ruling.
#3October 4, 2004· Wizards of the Coast
It can target delayed triggered abilities. For example, a card that says "Whenever a player cycles a card, you may exile target creature. If you do, return that creature to the battlefield at the beginning of the end step." triggers when a player cycles a card and creates a delayed trigger that happens at the beginning of the end step. You can choose to target the "at the beginning of the end step" trigger when it is placed on the stack at end of turn.
No other card is named in this ruling.