A quality paint brush will have hairs that form a point and have a good spring to them - they bend while painting but return quickly to their original shape. A good brush will also hold lots of paint and deliver that paint evenly throughout the stroke.
Traditional glass painting brushes include many different types. Paintbrushes can have a number to indicate their size - the larger the number, the larger the paintbrush. Other brushes are sold by size in inches (2.54 cm per inch).
Some of the most common types of brushes for glass and ceramic painting include:
Ox hair tracing riggers.
A rigger is a round brush with longish hair, originally used to paint the rigging in painting of ships. The ox hair is strong and springy. These brushes are useful for long lines, excellent for heavy paints.
Cheap brushes will normally be of poor quality and the painted result will reflect that fact. High quality brushes, when properly taken care of, will give you years of friendly use and be well worth the investment.