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

Torpor Orb stops a creature's own enters-the-battlefield triggered abilities as well as other triggered abilities that would trigger when a creature enters the battlefield.

Replacement effects are unaffected by Torpor Orb's ability. For example, a creature that enters the battlefield with one +1/+1 counter on it will still receive a +1/+1 counter.

The trigger event doesn't have to specify "creatures" entering the battlefield. For example, Captain Storm, Cosmium Raider from the Lost Caverns of Ixalan release has an ability that says "Whenever an artifact enters the battlefield under your control, put a +1/+1 counter on target Pirate you control." If an artifact creature enters the battlefield under your control, that ability won't trigger. If a noncreature artifact enters the battlefield under your control, the ability will trigger.

Abilities that apply "as [this creature] enters the battlefield," including many abilities that ask a creature's controller to choose a color or creature type, are unaffected.

Look at the permanent as it exists on the battlefield, taking into account continuous effects, to determine whether any triggered abilities will trigger. For example, if you control March of the Machines, which says, in part, "Each noncreature artifact is an artifact creature," each artifact will be a creature at the time it enters the battlefield and will not cause triggered abilities to trigger.

If Torpor Orb and a creature enter the battlefield at the same time, that creature entering the battlefield won't cause triggered abilities to trigger.