Are they essential for lectures etc?
No. I'm not sure if I've ever seen anyone use a laptop productively in a lecture. I have seen people play games and watch anime on them though, which seems fun, but probably isn't very useful
Do many people have them?
Some, from what I've seen that depends a bit on faculty (As you may expect). In Informatics they're relatively common, but seem a little more thinly spread elsewhere.
Whats better- use folders or a laptop?
Personally I take notes on paper in lectures, it's far easier than messing around with a laptop, and in a crowded lecture it's far more convenient this way (For me at least). I purchased a laptop for uni because I travel quite a bit to get there and back each week, and where I stay isn't terribly secure when I'm not around so purchasing a desktop machine and leaving it there is not an option.
For me it's just more convenient. If you do decide to purchase a laptop, you may want to consider buying used in order to cut the price significantly. As ben7 said, 'known' brands, are generally a good idea.
I've had a laptop for I guess around two years now which I purchased used, it's nothing flash, but is more than adequate for word processing and uni work while I'm away. So far I'm yet to experience any break downs with it, so being used doesn't necessarily translate to poor performance.
The price on a lot of new notebooks does look rather daunting, but it's important to remember that if all you really want it for is uni type stuff, and maybe to play some music or some stuff like that, you won't need one of the top of the line $3000+ laptops (As nice as they would be to own
)
Good luck with it, post again if you need any more help and I'm sure someone will help you out.