Beyond quality, however, there's even more to choosing the right brushes. Each medium - oils, acrylics, watercolor - requires a different type.
You'll find both natural hairs and synthetics recommended for watercolor work. Natural hairs have an edge with their construction - they're responsive and reflexive (translating into having a "spring"). Handles are generally shorter for better control working on a flat surface. The two brush types you'll need are flat and round. (Rounds are those with pointed tips.) Choosing sizes is dependent on what type of painting you're creating - it might be a large watercolor or a miniature, for instance.
Brushes for Oil Paints
Longer handles are a general requirement for oil painting brushes. You'll be standing at an easel and need the extra length for better control. Natural hog bristle is a nice choice, but I also recommend synthetics. These are firm enough to produce finer details when needed. You'll need several sizes and types - filbert, round, flat - for detailing as well as for covering larger areas.
Brushes for Acrylic Paints
As with oils, brushes have to stand up to heavier loads. However, because acrylics are often mixed with water, synthetic brushes tend to work best. These will also have longer handles as most work is done at an easel. Because acrylic artists utilize many of the same techniques as oil painters, you can select from the same types and sizes.
With the right brush, even beginners can see their art come alive. This may be a case where those just starting out will do best using higher quality brushes.