Until Michael and Julie both mentioned they had no idea what Halloween was, I presumed that it was celebrated all over. Turns out it isn't, so I thought I'd give a short synopsis.
It's really an old Druid/pagan holiday, Samhain (pronounced Sow-een). The day when the veil is lifted between the worlds of the dead and the living. (And other ethereal phenomena.) Interestingly, Mexican and South American cultures celebrate it as "The Day of the Dead", about 5 or 6 days later.
The decorations are autumn-festival based, at least here in the Northern Hemisphere, (a festival which stretches into the end of November), with the addition of jack-o-lanterns (pumpkins carved up into spooky faces). People put votives inside the jack-o-lanterns at night.
In the US, and I believe in the UK as well (I'll have to check on that), the custom is to dress up as someone/thing else. The idea is that the "devil" won't find you because you are masquerading as someone else, heheh.
The adults have costume parties, and the kids dress up and go "Trick-or-Treating" from door-to-door, asking for candy. If the kids don't get their candy, they then perform "tricks" upon the householder. Unfortunately, this leads to some vandalism -- like toilet-papering the yard, smashing pumpkins, throwing eggs, etc.
Halloween took a dark turn about 20-30 years ago here, with some adults handing out poisoned treats, so we stopped being available for the handouts, not wanting to get drawn into that.
Some years I get the "spooky" spirit, though, and join in the fun!