It started as a pagan holiday to keep evil spirits away. It was called Samhain. It was thought to be the day when days got shorter for the year. More darkness meant more chances for evil spirits to invade (or so they thought).
I think it was supposed to be a day where you scare the dead or something. I read somewhere (Don't remember where) that it was a pagan holiday. It was influenced by all saints day and all souls day. Nowadays it's all about trick or treating though.
Halloween started as a religious holiday,and was in fact,supposed to be "All Souls" day,to honor the dead and their journey to the afterlife.Then a bunch of different cultures got a hold of it,and then the Commercial Costume vendors got a hold of it,and then the Advertising Agencies got a hold of it,and it has become the intense farce you see now.But its nice to see kids have fun with it.We just decided in my neighborhood this year to reject a lot of the horror and go back to pumpkins and spinning wheels and corn.It sort of looks nicer since we did that.
Originally it was the Pics thanking mother earth for the bounty. They would wear masks of animals and dance around a bond fire. When the Romans and the Catholic church took over what is now Great Britain as a way to convert the pics they chose to make a celebration at the same time. November 1 is All Saints Day or at the time it was a called All Hallowed Day. The night before became known as Hallowed Eve which as morphed into Halloween. Some say the spirits would leave the night before All Saint's Day and the gourds were carved an lit to keep the spirits out of people's homes.