A fibrous carbon composite. The individual strands are typically woven into a rough
cloth, then covered with
epoxy resin once it's been molded to the desired shape, much like
fiberglass.
It's one of the vaunted new composite materials to come into common use in the 1990s, and is extremely light and strong. Nowadays it's recognizable enough to be a fashion statement -- that dark grey, glossy, woven material you often see on automotive eye candy (spoilers, dashboards) is carbon fiber.
It's often used in automotive & motorcycle racing bodywork, because it's so much lighter and stiffer than fiberglass or aluminum. It's pretty brittle, though, so it's not used for stressed members very much. Only John Britten was crazy enough to try that, when he took carbon-fiber "bones" and wrapped then in kevlar "skin" to make up for the former's brittle nature with the latter's toughness.