They waited for hours. This means that they started waiting hours ago, and then the waiting stopped prior to the present time. They've waited for hours. This means that sometime, it's unspecified when, they waited. Maybe today, maybe earlier, but sometime. It's simply a statement of an activity that took place and is not ongoing. They've BEEN waitING for hours. This means that they started waiting hours ago, and they are still waiting.

No, this sentence could relate to "waiting" that is still ongoing. Example: A nurse says to a doctor: "Can you please see the last two patients who are waiting to be seen by you? They've been waiting for hours."

They've waited for hours a) for the bus but it is coming now b) for the bus and there is still no sign of it coming