composites
A composite is a material made by combining two or more different materials so the mixture beats either one alone. The oldest example is straw in a mud brick, or steel rebar in concrete: each partner covers the other's weakness. Fiberglass and carbon-fiber-reinforced plastic (used in boats, aircraft, and bike frames) are the modern high-performance versions.
A composite has a matrix (the continuous phase that holds everything together and transfers load - often a polymer, sometimes metal or ceramic) and a reinforcement (strong, stiff fibers or hard particles embedded in it). In carbon-fiber-reinforced plastic, thin carbon fibers carry the load (very strong and stiff along their length) while the soft plastic matrix binds them, spaces them, and passes stress between them. Concrete does the reverse trick: concrete is strong in compression but weak in tension, so steel rebar (strong in tension) is added to carry the pulling loads. You can estimate stiffness with the rule of mixtures - a weighted average of the two ingredients' properties.
The catch is that a fiber composite is strong ALONG the fibers and much weaker across them (it is anisotropic) - designers must aim the fibers along the load path, or cross-ply them. Composites give the best strength-to-weight and stiffness-to-weight of any structural material, which is why aircraft use them, but they are costly, hard to inspect, and very hard to recycle.
For the same stiffness along the fibers, carbon-fiber-reinforced plastic is about five times lighter than steel - which is why race bikes and airliner wings use it. Turn the load 90 degrees, across the fibers, though, and it is far weaker.
A composite: superb in the fiber direction, weak across it.
A composite is a physical mixture with a visible boundary between its parts, not a chemical compound or an alloy where atoms mix at the atomic scale. The whole point is that the ingredients keep their separate identities and each does its own job.