A contact is anyone with a profile in Ometria, regardless of their lifecycle status, or whether or not they are subscribed.
A customer is a contact who has placed at least one order.
Julie has subscribed to receive marketing communications from you, and her email address is in your database, but she hasn't bought anything from you yet. Julie is a contact.
When Julie places an order with you, whether valid or invalid, she becomes a customer as well as a contact.