The basic difference between the two is the density. Porcelain tiles have a higher density and are harder than their ceramic counterparts. This is due to their manufacturing process and material composition.
Porcelain tiles are denser than ceramic because they are made of finer clay, they are compacted at higher pressures and are backed at higher temperature, upwards of 1300 degrees Celsius. Ceramic tiles have less density, are not as hard and are more porous which make them absorb higher amounts of water.
The density of porcelain tiles makes them more durable, hence they last longer. In addition, they are impervious to water which makes them waterproof and stain resistant.
Because of these characteristics, they can be used outdoors. Porcelain tiles are perfect for high moisture and high traffic areas such as bathrooms, hallways, and kitchens.
Ceramic tiles have low resistance to frost, moisture, and stains; therefore, they are better suited for indoor applications.
The cost of porcelain tiles is higher than ceramic tiles, but they have longer life span.
Below is a highly informative article from Home Depot outlining these and other differences between the two:
Porcelain vs. Ceramic Tiles – The Home Depot