What are composites?

A composite is a material which consists a reinforcement phase (the part which provides strength to overall material) and a matrix phase which completely surrounds/binds the reinforcement phase. These two phases have to be dissimilar. The dissimilarity is usually very easily identifiable. A composite can even have more than two distinct phases.
Take the example of the fiber glass. Small glass fibers that can be easily identified provide the strength to the composite whereas a thermosetting polymer forms a matrix around these glass fibers. The glass fibers are embedded into the matrix. In fiber glass typically epoxy, vinyl ester or polyester is used as a matrix material. The matrix by way of surrounding the glass fibers protects them from the environment and is also used to transfer load to the fibers.
Consider a sheet of fiberglass which undergoes tensile force at each ends. Although the force is being applied only at the ends of the sheet, the load will be transferred to all glass fibers in the composite through the matrix.
Composite matrix can be a metal (metal matrix composite), ceramic (ceramic matrix composite) or a plastic/polymer (polymer matrix composite).
In the same way the reinforcement can be of different materials, but as the function of this phase is to provide strength, hence it is stronger than its matrix. The reinforcement can have different geometries. Sometimes reinforcements are employed as particulates or aggregates as those used to make concrete; they can be fibers (glass fibers in fiberglass) or reinforcement can come in the form of a structure such as mats.