Market segments are groups of prospects that share similar characteristics. Market segments could be industries such as manufacturing, health care, energy, etc., or much smaller segments within an industry. Segments could be entities in different industries but which share common qualities such as government contracting.
Look at your list of clients to see what similarities they share. As you identify entities with similarities, you will be able to identify other entities that share those characteristics.
Keep in mind that referral sources are also a market segment. Understanding your market segments will help you identify better ways to develop business in each.