Hallowe'en is a short form of 'all hallows eve' which means a holy evening, or the Christian festival of All Hallows Evening. However, its origins are much older than Christianity. It goes back to the ancient Celtic festival of Samhain, which was celebrated mainly in Scotland and Ireland. This festival was usually held on 30 October or 1 November and was intended to mark the end of summer and harvest time and the beginning of winter. It was also considered a time when spirits walked, an idea which has survived into today's festival.
In the early Christian festival of All Hallows Eve, it was common to go to people's houses and ask for 'soul cake' which was meant to help with praying for the souls of the dead. Trick or treating, which in England goes back a long way, may have started with this but it may be even older.
There has always been a connection with the idea of the dead being active at this time, and we can still see this in Halloween masks and costumes today. The idea of getting ready for winter can be seen too, for example in eating autumn foods like pumpkins. However, for many people it has just become yet another commercialised festival and a means to get people to spend more money.