Fiber reinforcement in thermoplastics has three basic forms; short fiber reinforcement, long fiber reinforcement and continuous fiber reinforcement.
In general, with short fiber reinforcement, the fiber length is on the order of 100 times the fiber OD. Since most fibers are much smaller in diameter than a human hair, short fibers look more like powder to the unassisted eye. Short fiber reinforced thermoplastics tends to be manufactured by mixing the fibers into the molten thermoplastic. The fiber length and random orientation within the matrix make it relatively easy to achieve a good wet-out (i.e. completely encase all fibers with matrix) with this method. When compared to long and continuous fiber reinforcements, short fibers composites are the easiest to manufacture, but offer the smallest increase in mechanical properties.
Image #1: A photomicrograph image of short fiber reinforcement; (used with permission from Victrex Polymer Solutions)
Short fiber composites tend to be formed into final parts with a molding and/or extrusion process (see "Processing Thermoplastic Composites"). These processes require that the thermoplastic flow predictably to fill in molds and/or dies, and the short fibers do not interfere (much) with that process.