Seder is the Hebrew word for order. This meal at Passover has an order to its rituals and to the way the foods are served at the table. There is a religious service before the meal that follows the liturgy in the haggadah, which means the telling in Hebrew. At most seders, all participants have a copy of a short booklet called a haggadah at their elbow, to follow the order of the seder.
The seder is set up like a dinner party in the ancient world. Reclining like Greeks or Romans, we drink wine during a seminar on the Exodus from Egypt before the main meal. (Most families accomplish this reclining by sticking a pillow in the service leader's chair, but you might get one, too.) Then there is a great feast with wine, and following the meal the singing of blessings and songs. The haggadah instructs the eating of ritual foods to help tell the story and set the mood.
The Guide to Passover for Interfaith Families is also available in PDF and Word formats.