Black tea fannings are usually small particles of black tea left over from the production of larger tea varieties, but are occasionally manufactured specifically for use in bagged teas.
Black tea dusts are the finest particles of tea left over from production of the above varieties, and are often used for tea bags with very fast, very harsh brews.
Black tea fannings and dust are useful in tea bags because the greater surface area of the many particles allows for a fast, complete diffusion of the tea into the water. Fannings and dusts usually have a darker colour, lack of sweetness, and stronger flavor when brewed.