Composite materials (composites) consist of several components with different properties and distinct boundaries between the components. The composites can be “filled” with some particles or short/chopped fibers or “reinforced” with long (endless or continuous) fibers. A matrix is a material that bonds the fibers together or is filled with short fibers. Typically, the matrix has much lower mechanical properties than the fibers.
Only composites with long (or continuous) fibers are actually structural materials because only long fibers provide high stiffness and strength!
So, further we will talk mainly about continuous fiber composites.