A filler (very low aspect ratios) is a component that occupies space and combines with other substances in a matrix but adds little or nothing to the strength of a composite. Fillers are usually much cheaper (e.g., ($0.30/kg) than strength imparting materials. Fillers (very high aspect ratios) may be used to add weight (e.g., marble dust to very cheap plastic carpets or chalk to large plastic flower pots). Fibers are relative expensive ($2.00 to $30.00/kg) components mainly added to a composite to impart strength (e.g., carbon fiber in a golf club). Some fillers (e.g., flax shive) have a moderately high aspect ratio and hence can be used as a "reinforcing filler" but this is not so common.

There was a typing error in my answer. The words "(very high aspect ratios)" should be after the word "Fibers" and not after the word "Fillers".