Halloween, really All Hallow's Eve, is a Christian festival (Catholic actually!) and is the night before All Saint's Day. A day to celebrate all the saints who do not have a special day.
Samhain is a Pagan festival when we remember and honour the spirits of our ancestors. In Pagan mythology it was thought to be when the physical world was closest to the spirit world; when the "underworld" could easily be contacted.It is also the start of the New Year,November 1st. Halloween is one of the many Pagan festivals "adopted" by the Catholic Church.