Paint brushes come in many sizes, shapes and styles. The bristles can be made up of either a natural hair or synthetic compound consisting of polyester, nylon or a blend of both. The bristles (hairs) can be either solid or hollow as well as being tapered or not. As a general rule the bristles that are tapered give a better finish.
Synthetic bristles generally last longer than natural bristles. Synthetic brushes are better for water based paints because they tend not to expand when they get wet where as natural bristles tend to work better for oil based paints and varnishes.
Professional painters generally find brushes that they become comfortable with that give a high quality finish by holding a good workable amount of paint, a constant release of paint from the brush as well as not dragging or showing up brush marks.
Good quality brushes are a must. It’s a bad option to go out and buy a cheap brush for your painting job. All a cheap brush is going to do is have the hairs fall out and leave you with a less than quality finish. Also with a good quality brush you will get a higher quality finish in a shorter time.