Halloween is a weird thing. I hear about people from other countries talking about it as an "American" thing. Why it is "American" is because it's comprised of bits and pieces of different cultures in the US.
The day itself is "All Hallow's eve", which was named by the Catholic church. But the date is actually of Celtic origins of their Samhain (Celtic New Year). They believed on that day that the living and dead would intermingle, so they would extinguish the "flames" in their house, and dress up as ghouls so that they wouldn't be bothered.
"Trick or Treating" comes from a 9th century custom called "souling" where people went from door to door gathering a fruited bread from people who lost family members in the year. They would then pray for each piece of bread that they received.
"Jack o' Lantern" comes from some Irish folklore about "Jack" tricking the devil into being trapped, then wandering around the Earth with an ember in a turnip (I don't remember the whole story).
So, though Americans celecrate it, the holiday has many beginnings, none of which are "original" to the US.
