There's often a sea of paint brushes available at any art store. Did you know that some of the hairs and bristles are synthetic and others natural and that they each have distinct properties?
In some cases the synthetic brushes are obvious, the plastic strands are a bright white colour. On the other hand some of the synthetic brushes have dyed plastic fibres so they look quite natural. The dye breaks down the surface of each strand so that it is rougher (on a microscopic level) and this helps it to carry more paint.
Synthetic brushes come in soft and firm and have a good tension against the surface of the canvas while you paint a quality referred to as "spring". Unless they are very soft they also return to their natural shape quite readily, a quality referred to as "snap". Synthetic brushes come in a wide variety so that you can always have an inexpensive alternative to natural hair brushes.
Natural hair brushes can be expensive. They are made with animal hair, commonly, sable, and hog. Other brush hairs I have seen used are russian blue squirrel, badger, ox, goat, pony and camel. Each strand of animal hair has small divets (on a microscopic level) along the fibre. These help the hair to naturally carry more paint than synthetic bristles.
Hog hair is the most common of the firmer natural brushes. It has a tendency to create split ends overtime as it is used. These are called "flags". As more flags develop the brush marks will change slightly - in this stage the brush can create very controlled dry brush effects that look as meticulous as airbrushing! These brushes will continue to wear down until there is nothing left while still making an excellent brush. Stiffer brushes such as hog hair are commonly used with heavy body paints such as acrylic and oil because they can push the paint around without getting "gummed-up" like softer brushes do.
Of the softer hair brushes the kolinsky sable reigns supreme. It's from an animal in the weasel family found in parts of Russia and China. If you were to look at one of these hairs under a microscope you would see that it has a fine point at the end of each hair. From there the hair widens and then becomes slightly thinner again. Because of this reverse teardrop shape at the end of each hair there is a perfectly formed reservoir that carries the paint and delivers it to the needle fine tip. These brushes are highly recommended for water colour since thinner fluid will flow properly through them. They are amazing precision tools and there is way less "reloading" the brush with paint required.