Halloween isn't a Wiccan holiday. It was at one time a Christian holiday of All Hallow's Eve with the next day being All Saints Day. But now it is primarily a secular holiday largely promoted by the candy industry to sell more candy.
The holiday that Wiccans celebrate is known as Samhain (pronounced Sah-wen or Sah-ven) meaning " summer's end " in Irish Gaelic. We know that the ancient Celts celebrated a festival at this time of the year; however, since the Celts left no written records of their beliefs we don't know exactly what the festival entailed other than it seemed to involve bonfires and cattle. It marked the last harvest of the year when the animals to be killed and preserved for the winter were slaughtered. (This meat harvest had to wait until the weather was cold enough that the meat wouldn't spoil before it could be smoked or salted down.)
In modern Wicca the celebration of Samhain isa complex one. Mythologically it represents the Goddess as Crone going into the Underworld to seek her Lord that is amongst the Shades. With her leaving the Earth, the weather will turn colder and the fertility of the earth will abate. Next it is still considered the last harvest festival even though most of us have little to do with farming anymore. But it is also considered perhaps the most sacred of our Sabbats as it is the time when we honor our Ancestors and our Mighty Dead. We view it as a time when the veil between the world's is thinnest so the spirits of our ancestors are close to us. (This idea is also reflected in the Mexican Day of the Dead.) A common way of celebrating is with a "Dumb Supper" There are various ways of celebrating a Dumb Supper. In the coven in which I work we set up an altar in the place where we will be eating. On the altar we place pictures of our loved ones who have passed on, then fill plates with their favorite foods and put those in front of the pictures. That way our Beloved Dead can be with us during the feast.
Divination is thought to be especially potent at this time of year. We will typically each read our divinatory tool of choice while in circle to see what it is likely the coming year will bring to us.
There is some excellent information on the history of Samhain and of Halloween at http://www.witchessabbats.com/index.php?option=com_content&view=article&id=18&Itemid=29
And of course to our Folks in the Southern Hemisphere, October 31 marks Beltane rather than Samhain as the seasons are reversed there.