Looks like there are some great posts below. I just wanted to add that we have experienced night terrors with my 19 month old as well. He had about 3 episodes when he was 15 months.
If his screaming in the night gets worse when you try and comfort him, it is probably night terrors. This is very common and normal. A change in schedule, being sick, teething, or lack of sleep can all increase the possibility of these episodes happening. If the episodes are happening at about the same time every night, it can help to wake him up about 10-15 minutes before he has one.
If you can't catch it and if it really is a night terror it is best to not try and comfort him, because even though it goes against every motherly instict that you'll have, comforting him will make it worse and prolong the episode.
If you try and talk to him, be very direct. Sometimes direct firm instructions to lay back down and go to sleep will work. If not, something that worked for my son was to sit on the couch with him and turn on his favorite video. It would give him something to focus on and then he would calm down and finally fully wake up.
Good luck :)