Clay mud, when dried and then "Fired", melts together forming a solid, hardened mass. Over the centuries people have learned to form the clay into tiles (thin square or other shapes) to be used for decoration or protection. Tiles can be painted using a colored, powdered glass compound, then "Fired" again to cause the glass to melt to the surface of the tile. The "glazed" surface resists moisture and staining and works as a floor or wall surface because of those traits and the fact that it doesn't wear out very fast.