A customer can both be internal customer or an external customer. An external customer is a buyer of your services and products but external to your organization. An internal customer is one who is connected to your organization and is internal to your organization.
An example of your external customer could be people buying your products in the marketplace. Internal customers are your shareholders, employees & other stakeholders. Customers can also be an intermediate customer or an ultimate customer.
An intermediate customer is the one who purchases the goods for re-sale. For example, your dealers are your intermediate customers. An ultimate customer is one who does not re-sale the goods or services obtained. The customers purchasing from your dealers are your ultimate customers. Although sometimes used interchangeably, there is a subtle difference in consumer and customer.
A consumer is the person who actually consumes the products or services, whereas the customer is simply the purchaser & need not be the consumer. The intermediate customers are never consumers as they buy for re-sale. The purchaser of industrial goods & services is called industrial customers or business to business customers.