Natural bristle filaments are made from animal hair, commonly from sable, squirrels and hogs. These brushes work best with oil-based, alkyd paints, stains and varnishes. They don't work well with water-based paints because the bristles get wet the same way hair does, and so become limp and less effective. Natural-bristle brushes also don't work well on rough surfaces, which can break the bristles' flagged ends.
Artificial-filament brushes are made from nylon or polyester, or both. They work well on rough surfaces and with any kind of paint. In addition, nylon-filament brushes are durable and generally less expensive than natural bristle brushes. Polyester filaments are popular with do-it-yourselfers because of their low price. They retain their stiffness better than any other kind of brush. The drawback is that because they are a stiffer-type brush they have less flex and leave more brush marks on surfaces. Also, they are not as easy to clean as nylon.
Nylon/polyester blends are the most popular synthetic paintbrushes since they combine the best qualities of both for great performance. The polyester bristles are used in shorter lengths for stiffness when using water-based paints; nylon bristles are used in longer lengths for precise tipping and durability during use.