Synthetic fibers are made by a chemical manufacturing process. Polyester is the most common of these. In most cases, a synthetic fiber has greater strength than a natural fiber. The fibers of thread can be staple length fibers or filaments fibers. A filament fiber is a continuous fiber and can be very long.
Silk is one continuous filament fiber.
Synthetic fibers are produced as continuous filaments and then cut into desired staple lengths. Thread made from longer fibers, staple or filament, tend to have greater strength than short staple fibers. Short staple fibers are often used in inexpensive threads, causing them to be fuzzier and weaker than when longer staple length fibers are used. These shorter fibers often loose lint in the sewing process and my cause machine problems. No one thread can be used successfully on every fabric.