Halloween began as All Hallow's Eve which eventually morphed into the word Hallowe'en (or Halloween). It is the day before All Saints Day, a Christian feast celebrating all martyrs and saints. A similar celebration, called All Soul's Day, which lands on November 2nd, is said to be where most of Halloween's traditions originate. Children and the poor, hoping to comfort dead souls, traveled to wealthy homes and asked for alms and "Soul cakes". Jack-o-Lanterns and trick or treating in costumes are American originated and have since spread around the world.
Today, Halloween is a beloved holiday for children to dress up in scary/spooky/silly costumes and go door to door asking for candy and treats. Other modern traditions include attending dark themed parties, carving pumpkins into Jack-o-Lanterns, watching monster movies, and unfortunately, vandalism via egging and destroying halloween props. Secular celebration has largely overshadowed the religious celebrations and most people either don't know or don't care about its origins.