Its axis of curvature coincides with that of the inner conductor, hence the name coaxial. Finally, an outer plastic cover surrounds the rest. Coax comes in several sizes. Standard Ethernet cable, the yellow stuff called thick Ethernet, is about the diameter of a man's thumb. Thin Ethernet, the black cable, is about as thick as a woman's pinky finger.
Thicker coax is more robust, harder to damage, and transmits data over longer distances.
It's also more difficult to connect. This combination is bulky and expensive. Unshielded twisted-pair is used extensively in the nationwide telephone system. Practically every home that has telephones is wired with twisted-pair cable. In the past few years, vendors have been able to transmit data over twisted-pair at reasonable speeds and distances.