As far as head shapes, brushes are offered in a range of shapes which allow for different types of marks to be made. Depending on the range, shapes include round, long flat, filbert, short filbert, short flat/bright, and fan.
Brushes come in different sizes and these sizes are given numbers. However, each number does not necessarily equate to the same size brush in different ranges, and this is particularly noticeable between English, French and Japanese sizes. Consequently, if you are choosing a brush it is important that actual brushes are compared rather than simply relying on the sizes of the brushes you currently own.
Handle length varies as well. If you are a watercolourist then it is likely you will work closer to your painting, making a shorter handle a good choice.