In Australia, up until about five years ago it was just something in American cartoons that no-one really understood, but now it has been picked up as being something saleable, so we've now got all the time-wasting and money-spending and annoyance, but none of the tradition.

And still no-one really understands it.

Except me. I googled it. It seems to be pretty much as catsPHiddle described it.

I also learned this:
Jack O'Lantern was an Irish kid who tricked the Devil into climbing a tree and then he carved a cross on the trunk and wouldn't let the devil down until he promised to stop tempting Jack.
When Jack died he couldn't go to Heaven because he had dealt with the Devil and he couldn't go to Hell because he had made a mug of the Devil, so he was left to wander the empty darkness eternally, and for this purpose was supplied with a candle. (By whom? And what did he expect to see with it I wonder?)
To shelter his precious candle he stuck it in a turnip, but apparently when the story arrived in America (or Jack himself?) pumpkins were more plentiful than turnips. Easier to carve too, I suppose.

I like that story, but Halloween itself, celebrated today, the way it's celebrated, yeah, that's rubbish.