My 3.5 yo DD has them too, they are very upsetting to witness aren't they? 
But like Sleepless' DS, she doesn't remember them, which is reassuring. We are normally unable to wake her, so just tend to sit with her, talking quietly and ready with a hug if she does wake. If we try to cuddle her in the middle of it, she fighs us off.
There's some really useful stories and advice on the sleep boards. From there I learnt that they are commonly linked with tiredness, and to be really strict about naps and early bedtimes to make sure your DD doesn't get overtired. That has worked for us, as we were getting into a vicious circle - her disrupted sleep made her more tired, cue more terrors, cue even more tired, and on and on.
My HV suggested that at this age, children aren't able to articulate a lot of what they see - and events, like OhNo mentioned, can't be processed verbally and so are processed as dreams instead. If its hard for a 3 yo to do this, then it must be doubly hard for your 18mo. We try to take 10 minutes after tea each night to talk about what she enjoyed today, if anything made her sad and why it's ok now, to try to help this processing. And then after a story we talk about what we're looking forward to tomorrow. Whether it is the improved sleep, or the chats, or just growing out of them - she is better now than even just a few months ago.
I hope your DD grows out of them soon