Carbon fiber composites currently offer the best weight, stiffness, durability and ease of use parameters out of all other materials available for bicycle component manufacture. Composite literally means made of several materials - in this case carbon fibers, and resin.
Carbon fiber layup and the quality of manufacture is extremely important in being able to control the torsional loads. Unlike isotropic materials, carbon fiber composites are a man made material that can be made to satisfy virtually any mechanical property without changing the overall structural shape of the part, they are entirely anisotropic.