Bristle Type: Paintbrushes are made with two basic types of bristles, sometimes called filaments, natural and synthetic. Natural bristles are made from some sort of animal hair, such as hog or badger. Synthetic bristles are often made from nylon, polyester, or a blend of the two. Natural-bristle brushes are best for applying oil-based paints and synthetic-bristle brushes are recommended for water-based paints. However, some synthetic brushes can be used to apply both latex and alkyd paints so be sure to check the packaging on the brush to ensure it's compatible with the paint you're using.
Bristle Shape: Most paintbrushes available today are square-cut brushes. They're perfect for holding and laying paint onto virtually any surface. However, square-cut brushes don't provide as much control when painting into corners, up to adjacent surfaces, or along narrow edges or surfaces. For more precise control, use a sash brush, which has its bristles cut at a slight angle. Sash brushes are particularly well-suited for cutting in around the perimeter of a room.
Bristle Tips: Better quality brushes have bristles with flagged, or split, ends. Flagged bristles hold more paint and spread paint more smoothly. Some brushes, especially sash brushes, have tipped ends, which should not be confused with flagged ends. Tipped brushes come to a point; they're not cut flat and straight, as is a standard brush.