A composite, or composite material, is something made up of multiple materials that meet two criteria. Firstly, the two or more materials have different chemical or physical properties. Secondly, the combined material has different properties to the individual components. A composite that includes a polymer can often bring additional strength in a relatively light material.
A polymer can be either a natural or synthetic (artificially created) material. It's key characteristic is that its chemical structure consists of a sequence that is repeated in a chain. Natural examples include silk and wool, while synthetic examples include PVS, polystyrene and nylon. A polymer composite can technically consist of any material combined with a polymer, but it most commonly refers to a composite where a fibre material such as glass is embedded in a polymer.
As with any composite, the main advantage of a polymer composite is that it combines the different qualities of its components. Scientists and engineers creating a polymer composite will look for a combination that brings together qualities that are both positive and wanted for a particular purpose. One common technique is to combine the tensile strength of a fibre (meaning it can resist a "pulling apart" force) with the compression strength of a polymer (meaning it can resist a "squeezing" or impact force). Another goal can be to get this strength while keeping the composite material light.
Polymer composites are particularly popular in aerospace and automotive use where the combination of well-rounded strength and lightness is important. They can also aid manufacturing because they can be made in a single large, moulded shape. This reduces the number of parts that must be made, along with the number of connections such as screws; fewer connections mean fewer possible areas of failure. Polymer composites can also be used for strengthening buildings and bridges made from more traditional materials such as metals and stone.
The main drawback with a polymer composite is that you do not only have to worry about either the polymer or the fibres failing to stand up to force. If the force stretches the polymer more than the fibres, or vice versa, there's a risk that the polymer and fibre may become detached from one another. Another problem is that not all polymer composites stand up well to use in very high temperatures; those that do may be more expensive.