Are these actually active noise cancelling, or even noise cancelling
at all?

http://www.buy.com/prod/philips-acti...tml?adid=17992

Their list price is only 30 dollars.

I thought to be noise cancelling, there had to be a microphone that
listened to background noise and a circuit that reversed the noise and
simultaneously played it through the speakers or earphones.

It's not just on this shlocky website that I see the term used to
mean, afaict, "fills your ear canal so the noise can't get it". Or do
they do more than that.

Thanks.