Fire bricks 
Firebricks are manufactured in ways similar to ordinary masonry bricks but they are fired at higher temperatures until partial vitrification has occurred. They are more dense, which makes them more resistant to abrasion, they have have a lower thermal conductivity, absorbing heat but not transferring it to surrounding materials and a high thermal mass, enabling them to retain heat which radiates back after the heat source as been extinguished, in an oven or masonry stove, for example.
Different names - same product
Firebricks are known by various names including fire-clay, chamotte, refractory and fireplace bricks. They are made from clays which contain a high level of silica and alumina with traces of iron and manganese. Firebricks are graded according to the alumina content, which can range from 18% up to 90%. As the percentage of alumina increases, so the bricks have a higher density and can withstand higher temperatures. The correct grade of firebrick is determined by the temperatures required for each particular application.