Ceramic tile is made of clay and shale that has been baked and hardened in a kiln. The clay composition, in combination with the firing process, and whether the tile is glazed or unglazed, creates one of four basic tile types: Glazed ceramic which is durable; Quarry is unglazed and extremely durable; Porcelain can be glazed or unglazed is extremely durable and hard and can be used indoor or outdoor and Terracotta which is unglazed and fragile and requires a surface sealers during the manufacturing process or applied on-site during installation. Glazed tiles have a surface that provides color, design, ease of maintenance and are more resistant to stains. Unglazed tiles stand up better to war and more slip-retardant.