Paint brushes are a tool for the painter. There are high quality, expensive brushes and lower quality, less expensive brushes. So which ones are right for you? The answer is that it depends on what you're doing with them.
Cheaper brushes can be purchased by a set for a few dollars and can (should) be parts of any painters tool set. You do not want to use a $30 brush to mix paint for example. Because the cost is lower they can be sacrificed.
More expensive brushes should be reserved for finer detail work and cared for regularly.
As a general rule synthetic tip brushes are usually less expensive than natural hair brushes.
Below are some tips to get you started.
1. Always clean your brushes. Paint left in the bristles will dry and set. Once this happens the brush will no longer be useful for painting. They may still be used for other tasks such as spreading basing materials or removing paint from a pot. Your brushes should also be regularly cleaned with a brush soap. Brush soap will do a better job of removing paint from the bristles and can also condition the tip.
2. Never set the brush on its bristles. This will ruin the brush quickly. While painting if you're done with the brush rinse it thoroughly with water and either set it on it's side or store it bristles up. Some brushes come with a bristle protector so they can be stored with the bristles down.
3. Avoid getting paint in the ferrule. The ferrule is the, typically metal, part that holds the bristles and defines the tip. If paint gets in the ferrule it's difficult to clean properly and will lead to split brush tips.
4. Avoid hitting the ferrule on the water cup. This could lead to the ferrule crimp failing, becoming loose.
5. Use the proper brush for the proper technique. For example avoid dry brushing with a round brush.