A customer is anyone paying for a good or service. Without customers, businesses would go out of business.
When you're paying for just about anything, you're a customer. People shopping at the grocery store, sitting in coffee shops, going to movies, and buying DVDs online are all customers. The point of advertising is to drum up new customers for a business. There's a cliché that "The customer is always right," which is obviously not true, but shows how businesses try to make customers happy. They'd better. Without customers, any business is in deep trouble.