Most synthetic fillings are made of polyester fiber, a polymer based chemical. For most bedding products, the fiber is turned into long strands like fishing line and then cut into small pieces. Typical sizes of strands are 2" long by 6 denier round. Different types of strands are on the market. Solid core fibers are the lowest cost to produce, being as the name suggest, one solid round fiber. Hollow core fibers are made with a hole in the center of the length of the fiber, this produces a lighter more resilient fiber (better bounce back ability). The strands are turned into fillings for all types of bedding products. Either fiber can have a straight type length or a conjugated (curly) appearance.
Fibers are either dry or slick, dry having no additional treatment after manufacturing. Slick fiber has a silicon treatment for a simulated silky feel. Combinations of slick and dry fibers are mixed to produce end products results such as softness or firmness. A mix of 50% dry solid core fiber with 50% conjugated slick hollow core fiber in a standard size 20 oz bed pillow would yield a lofty pillow with firm support.
My personal favorite polyester is Puffball (cluster) fiber. It is made of high grade polyester fibers, a special machine forms the polyester strands into a tiny ball about the size of a pea. Thousands of the little puffballs are blown into a product. It gives a down like feel to bed pillows and comforters; lightweight, fluffy, warm and resilient. People who are allergic to natural down should try puffball fiber.
Other synthetic fibers in use are polyolefin. Not as widely found, but a good durable fiber.