Not the same as a nightmare, Night Terrors are the next step up. They are when you a struck by extreme terror and panic (I think of it as an anxiety attack) while sleeping. Most of the time they happen early on in your sleep and when you wake up from them you are confused and scared, the world seems foreign. They are related to sleepwalking (somnambulism) and take place during deep sleep. Children between the ages of 4 and 12 are the most likely to suffer from night terrors. Adults who suffer are most likely suffering from some sort of psychopathology caused by abusing drugs/alcohol or a plain old stress or mental disorder.
They are next level shit. I suffered from them for about a year and the expirence was unreal. Dreams of pure evil and fear materializing in different forms (a 3 year old girl in her pjs with the features of a withered old woman floating outside the window). Long dreams of paralysis while evil hovered above you and you're trying to scream or move or somehow get somebody to wake you up but it feels like your brain can't send the messages anymore (it really can't). Everything being covered in fear (sort of like Nausea by Jean-Paul Sartre). Waking up in dreams over and over again until you think you’ve lost the ability to truly wake up.
I’ve met a bunch of people who suffered from the sleep paralysis. Also others who have the dream of all their teeth getting weak and loose and then falling out. It's weird. Definitely a message that you need to do some assesment of your mental health, stress levels or love of the drug you love.
info from The nightmare: the psychology and biology of terrifying dreams by E. Hartman and some double checking courtesy of www.aafp.org