The child cries, calls out, wakes up and eventually gets up to find his parents. If you hug him or talk to him, he wakes up, realizes what has happened and goes back to sleep relatively easily. After a nightmare, the child is able to recount his dream or at least explain the associated feelings ("I was scared, there was a wolf, it was dark...").
Finding the causes
Observe carefully: do the nightmares occur every night? Has your child got a very disturbed sleep pattern? If these signs seem to take hold for more than about ten days, ask your doctor for advice. There's maybe a concealed conflict, a family problem which, unadressed, is expressing itself in nightmares. Simply bringing the problem up with a doctor, in front of the child, may be enough to unblock the situation. Otherwise, a consultation with a child psychiatrist may be necessary to determine what action to take. Above all, if there's a problem don't just wait for it to go away: the situation could only get worse.