The reinforcement is usually much stronger and stiffer than the matrix, and gives the composite its good properties. The matrix holds the reinforcements in an orderly pattern. Because the reinforcements are usually discontinuous, the matrix also helps to transfer load among the reinforcements. A particle has roughly equal dimensions in all directions, though it doesn't have to be spherical. Gravel, microballoons, and resin powder are examples of particulate reinforcements.
Reinforcements become fibers when one dimension becomes long compared to others.
Most fibers are only a few microns in diameter, so it doesn't take much length to make the transition from particle to fiber. With either particles or short fibers, the matrix must transfer the load at very short intervals. Thus, the composite properties cannot come close to the reinforcement properties. With continous fibers, however, there are few if any breaks in the reinforcements. Composite properties are much higher, and continuous fibers are therefore used in most high performance components, be they aerospace structures or sporting goods.