The built in fireplace has a double shell design. The inner shell is usually refractory lined. The space between the inner and outer shell is used as a sort of heat exchangeer. It recovers heat and sends it back to your room, and keeps the outer shell cool, allowing zero, or near zero clearance. As a result, these units are much more efficient at recovering useable heat for the living space, as compared to a conventional fireplace.

The thermal efficiency is not as high as a wood stove insert, but each has its place. The Zero fireplace, usually has doors that close and prevent lots of interior air from be convected up the flue, but it is not air-tight like a wood stove. Combustion air is not as controlled as wood stove. It is more controlled than an open hearth fireplace.

As dodegeman said, the refractory bricks are replaceable, and 20 to 30 year life is not uncommon in these units. They are much easier for repair or replacement than masonry.

Hope this helps. Post back if you have more questions.