The same liquids that go in the dough. When the pizza is placed in the oven, the dough begins to 'sweat'. The water wants to go out and fats are melting. In this proces the edges of the pizza exposed to open space turn into a nice crust, but when the cooking surface doesn't allow these to travel freely across the skin of the dough because it there is no absorbtion it burns black.
When you fry meat or vegetables, you use oil or fat to provide the space for the fluids to come out en travel across the skin, and you get a crust by caramelization.