A market can be thought of as the collection of contexts in which you might sell your product. You can split your market into a set of market segments. Each of those segments represents a group of customers, each of whom shares a set of problems for which they would pay for solutions.
Scott Sehlhorst at Tyner Blain offers a nice article on markets and segments of customers. The key (of course): customers who have a problem that willing to pay to solve.