Flock is a soft, velvet-like heat transfer vinyl that is thicker than your standard vinyl, making it the perfect alternative to embroidery. It gives your design a raised look as well as a textured feel. The soft touch makes it a great product to use for children or baby garments too. It comes in a variety of standard and bright colors, making it another popular choice if you are trying to achieve a retro design.
Source: Brooke Hestdalen
Flock starts as long braids of super-fine, spun fibers of the natural substance rayon. These strands have the appearance of beautiful silvery or golden hair. They are wound through a series of guides where they literally end up on the chopping block and are cut to various lengths ranging from around a quarter inch to a few microns. These tiny fibers go through numerous steps. First, they are washed to remove any impurities. Next, they are dyed to a wide range of colors. Afterwards, they are treated to give specific electrostatic properties and then dried, packaged, and shipped for application.
Source: David Lewis