Are My Peers (the people I work with) Internal Customers?
In the end it doesn't matter if you call some people customers or not, provided you understand the relationship between you and those people. Sometimes it's easy to get lost in semantics when it doesn't much matter.
Technically, your co-workers are NOT your customers, even if they consume what you produce (e.g. information, memos, etc.), because there is no EXCHANGE of value for any specific services or functions AND you are your colleagues share a reporting structure (common boss, if you want to simplify it).
That doesn't mean you don't interact with them as if they are important, but ultimately they are not internal customer.
So Who Are Internal Customers?<
Some people are customers of the internal type, even if they aren't paying you directly. Here are some examples:
HR provides services throughout the organization and most of its internal interactions should be considered as dealing with internal customers. The important part here is that they report through a different authority line. They are not managed by the same people as are managing the other parts of the organization. In effect, because they are a somewhat independent entity, it makes sense to consider them much like any external organization that offers services, because they have to treat those they serve as customers.
The same argument holds for IT (Information Technology) Departments. They report to different people than those they serve, and function similarly to any external IT consultant, and need to use the same consultative approach as externals would.