Anybody know how does opencascade discrerte and display a model?

I want to display a model using opengl and opencascade. I wrote some code but when i read a iges file, it is very slow. I want the model displayed in two ways: shading and wireframe.
But i don not know how does opencascade discrete the shape and how does opencascade get shape
data? Is there anybody who knows this ?Please help me!