For most of the painting applications you are likely to encounter, there are two general categories of bristles: natural and synthetic. For best results, you want to match your bristle selection to the type of finish you're applying.
Synthetic bristles, also called filaments, are typically made from nylon or polyester.
They are actually designed for water-based paints, since the synthetic fibers are not affected by the water and will therefore hold their shape.
Another synthetic fiber used in brushes is Chinex, which is manufactured by DuPont. Chinex fibers simulate the characteristics of natural bristles and therefore work very well with oil-base paints, but since they are synthetic they will stand up water-base products as well.
If you're not sure what type of paint you'll be using, or if you only want to invest in one brush that will work for different paint projects, there are also several synthetic brushes that use a combination of two or three different fibers, and are considered suitable for all paints. Natural fibers used in paintbrushes are most commonly made from hog bristles or ox hair. China is one of the leading sources of this type of hair, and therefore these types of natural fiber brushes have come to be known as China bristle or China brushes.
Natural fiber brushes are soft and apply paint very smoothly and evenly, creating a more uniform finished product. Since these are natural fibers however, they will absorb water and become limp and shapeless, which is why they are not suitable for use with water-base paints.
Size and shape
The other things you'll want to take into consideration when selecting a brush include its size, which refers to the brush's width - the shape of the brush, and the type of handle it has.
A brush's width should be appropriate for the surface you're painting. It would take forever to paint a house with a 1-inch brush, but a 4-inch brush would be quite a bit of overkill for painting delicate window trim.
Brushes also come with a square end, which hold more paint and works best for larger, flatter surfaces, and angled ends, which give the painter a little more control and are considered better for tasks such as painting trim and applying stains and clear finishes.
Professional painters will typically have an arsenal of a dozen or more different sizes and types of brushes available to suit whatever paint projects they're likely to encounter. For the homeowner, a good selection might include a 1- or 1.5-inch angle-tip for trim and moldings, a 2- or 2.5-inch angle- or straight-tip for general purpose use, and perhaps a 3- to 4-inch straight-tip brush for larger projects such as exterior siding and deck staining.
Most professional-grade brushes have wooden handles, which are considered easier to grip and less likely to slip out of your hand. Plastic and acrylic handles are also available. Handles also come in various lengths, thicknesses, diameters, and shapes, and should be selected for comfort and to suit the task at hand.