Bitter herbs, some people use romaine lettuce, some use horseradish, some people mix them. Charoset, a thick tasty mixture of ground apples, dates, nuts, cinnamon, ginger and wine. A few copies of the Hagadda, with an English translation. Those are the bare bone minimum requirements for the seder. Food, like chicken and potatoes are recommended.
Most people have a special seder plate on which the various item are displayed.
Three whole matzos are placed on the table. Many people have beautiful and special dishes just for this one evening. The matzos are covered with a cloth. During the seder, they are uncovered and recovered several times. The chairs are arranged to permit the participants to recline while eating.