Flock is one of the main things we use to detail the bases of our miniatures and terrain. There are different types of flocks, but are typically Static Grass (those short hairy fibers seen all over GW terrain) and the more standard powdery flocks used by railway modellers; such as those produced by Woodlands Scenic.
Static Grass is a rayon fiber that when charged with static electricity stands up to give the impression of standing grass. Many people simply use this a standard flock and do not bother to use a static applicator to make the ‘grass’ stand up, however the effect of properly applied static grass is wonderful.
The other variety (sometimes called turf) is typically made from ground foam (such as that in your cushions or cheap mattresses) that has been dyed certain colours to represent soil, burnt grass, weeds etc. Whilst it’s possible to grind your own foam to make better quality flocks, this video shows you how you can quickly and cheaply make the powdery flock using paint and sawdust.