Typically they are insulating and resistant to high temperatures and harsh environments. They are very large molecular structures. Usually they are low density and are not stable at high temperatures. Semiconductors have electrical properties intermediate between metallic conductors and ceramic insulators. Also, the electrical properties are strongly dependent upon small amounts of impurities.
Composites consist of more than one material type.
Fiberglass, a combination of glass and a polymer, is an example. Concrete and plywood are other familiar composites. Many new combinations include ceramic fibers in metal or polymer matrix. This type of description is too general to provide much insight into where and how materials are used and certainly not much about what a materials person does. In reality, iron has been important because we can change its properties by heating and cooling it.