Earthenware is a type of ceramic made from ordinary clay (as well as silty clay, sandy clay, or clay loam). Much like green glass (and clear glass, to a lesser extent), earthenware items can be produced in unlimited quantities as long as you have a source of clay in your fortress. They are brown in color and have a material value of 3, equal to that of obsidian.
Other than its potentially abundance, earthenware are also very light, making it perfect for crafting containers. However, earthenware containers must be glazed in order to store liquids.