What is a market?
A market comprises all the entities (people, businesses, governments) with the interest and means to buy a product. Having "interest" means that the entity perceives value in the product. Have the "means" indicates that the entity has access to the product and, if required, can pay for it. "Customers" are the part of the market that has purchased your product.
Unfortunately, the terms "market" and "customer" are often used interchangeably. In product management literature, there is often undue emphasis on understanding customers, often at the expense of understanding the part of the market that hasn't yet bought a product.