A much finer clay is used to make porcelain tile. Feldspar, quartz and kaolinitic minerals are covered by a spray. The clay is dry pressed and is fired at a higher temperature than ceramic tile. It's resistance to moisture is the most important characteristic of porcelain tile.
Porcelain tile is also resistant to frost, deep abrasion, and wear. Those characteristics make it an excellent product in harsh climates. At one time, porcelain flooring tile was only available in 12" X 12" squares. Today, porcelain tile comes in many sizes. A 20" X 20" tile can be used to create a stone 'slab' look.
The most beneficial characteristics of glazed or unglazed porcelain tile are less porosity. This makes it ideal for kitchens and bathrooms where water can be an issue, and it's density that tolerates higher loads.