Night terrors are when the child isn't awake and while can hold a convo has no recollection later. Some kids can't even be touched during one - it just escalates for hours (and hours and hours ) Waking them prolongs the terror. Because they are in their fight or flight mode and it's scary and confusing when their brain becomes conscious.
Usually at the same time every time. With DD1 they were between 11.30 and 1am every time, DS about an hour and a half after going to sleep and DD2 is only one sleep cycle after going to sleep.
Will come in waves. So, seemingly random and working up to more and more frequently. At a bad stage with DS they were several times a night. Then they work down to fewer and fewer until it all starts all over again.
It's their brain not able to switch sleep cycles.