They are available in numerous colours, patterns and shapes and are made of natural clay, decorated on one side with coloured glaze. The surfaces of tiles are normally glazed, while other tiles are matt or have an unglazed finish. Unglazed tiles must be sealed with the correct proprietary sealant (recommended by the manufacturer) once laid. The stronger tiles are thicker and darker on their underside and often come in larger sizes. They are fired at a higher temperature (vitrified) so that the particles fuse together. This results in the tiles being almost unbreakable when laid. Qualified tilers will fit any tiles in your new bathroom or new kitchen in no time.

Tiling on Wikipedia