Oyster Crackers are round, hard crackers meant to be added to bowls of soup at the table in which they soften.
They are very small, generally just about 1/2 inch round (1 cm.) They are made from flour, water and just a touch of salt in the dough. No leavener is used. They taste like saltines without the salt.
They are not made of oysters. The name probably came about from their being used in chowders and soups.