All right, you spoilsport. Halloween, also known as All Hallows' Eve, All Saints' Eve, or Samhain, is a celebration observed in a number of countries on October 31, the eve of the Western Christian feast of All Hallows' Day. It begins the three-day observance of Allhallowtide, the time in the liturgical year dedicated to remembering the dead, including saints (hallows), martyrs, and all the faithful departed.
A number of ways to celebrate have evolved over time, including bonfires, bobbing for apples, divination games, playing pranks, visiting haunted attractions, and telling scary stories. There's also carving pumpkins (or turnips, depending) into jack-o-lanterns, based on the Irish-Christian folktale (the lantern represents a soul unable to go to Heaven or Hell), and trick or treating. That's said to originate from the medieval concept of souling. Soulers (mainly consisting of children and the poor) would go from door to door during the days of Allhallowtide singing and saying prayers "for the souls of the givers and their friends". They'd be given soul cakes at the homes they visited. Trick or treaters today do much the same, and receive candy in response to the question "Trick or treat?" The implication, of course, that anyone who fails to provide a treat will be, in some way, tricked.
Like any holiday, you'll find some who cling more tightly to the religious meanings and some who enjoy it as a purely secular celebration, but it does have meaning beyond 'dressing in a costume and picking up candy from strangers.'