Before discussing the function of customer service, it is useful to clarify what is meant by a customer, with regard to a business. A customer can be a person, company, or department which makes use of the goods and/or services produced and offered by another person or organisation.
Customers come in two very different and distinct types, internal customers and external customers.
Your internal customers are other people or colleagues who are directly connected to your organisation that want or need something from you.
Your external customers are people from another company, or members of the public, who want to get information or buy products or services from you. The external customer may be the ultimate consumer of your businesses products/services (what most people would understand by the term customer) or another company your business supplies or someone who is directly affected by your business.