Who are your customers?

It seems like a straightforward question: When you work in IT, who are your customers?
We use the term all the time. We talk about customer satisfaction, customer relationships, customer service and customer experiences. Our understanding of customer-supplier relationships looms large when we set priorities and decide what to do and how to behave on the job.
The problem is that we’re never completely clear about who is and isn’t a customer. Whenever I ask a group of technical folks