What is internal customers?

Internal customers are people or groups of people within your company, that receive and make use of the products produced by you in your work function, or make use of a service provided by yourself. Example: i am the secretary of a law firm, and the lawyers (internal customers) make use of my service to prepare documents and make telephone calls. Similarly in a cellularphone manufacturing company, the procurement departments purchases materials required for production, in this case the internal customers are the workers making the cellularphones.