A night terror looks like the worst nightmare a person has ever had and is mechanically almost the opposite. Nightmares happen in REM sleep, they are dreams, and the sleeper can usually recount them. A night terror happens in deep non-REM sleep, there is generally nothing to recount, and by morning most people have no memory of it at all.
What is happening is a partial arousal. The systems governing movement and autonomic arousal surge into full activity while the higher cortical regions that would produce awareness stay asleep. The result is a body doing everything a frightened body does, sitting up, crying out, sweating, heart racing, running on a fear response with nobody home to interpret it. It is also why the person cannot be reasoned with during an episode: the part of them you would be reasoning with is not awake.
This is most common in young children, which is a mercy for the household in one respect, since most grow out of it, and no mercy at all in the moment. The standard advice follows from the mechanism rather than from folklore: do not try to wake them, which prolongs the confusion, keep the space safe, and wait. The episode ends on its own and the sleeper usually goes back down without surfacing.
Triggers are unglamorous and mostly modifiable. Sleep deprivation is the big one, along with irregular bedtimes, fever, stress and some medications, and there is a clear familial pattern. The practical upshot for a household in the middle of a bad run is that fixing the sleep schedule does more than anything that happens during the episode itself.