There are several methods of making carbon fiber but essentially it consists of first making fibers out of a carbon rich precustor material. The original size and shape of the fibre will remain in the finished carbon fiber, but the interial chemical structure will have been greatly modified through the various heating cycles. The first steps are carbonizing and stretching precursor fibers, either PAN: Polyacrylonitrile, Pitch or Rayon. There are several cycles of heating at varying temperatures excluding oxygen. This process drives off most of other elements (hydrogen and nitrogen mainly) of the starting material leaving carbon behind. It also allows the carbon to gradually crystallize in its characteristic honeycomb way. If you haven't seen it yet, go to my Carbon Research page and look at the video on Carbon fibre structure. it's fabulous.