Halloween was once the Celtic holiday of Samhain where on November 1st people would dress up in costumes and light bonfires to ward off ghosts. Halloween was also used to signify the end of summer and the harvest.
Halloween is now a celebration of spirits, monsters, and otherworldly things. People now dress up as their favorite person, object, or pretty much anything they chose. People also use Halloween as a time to have parties with no real reason other than to have fun ( I can't really blame them October is pretty boring).