You know their names. You've taken the time to know them personally. Some coffee here and a chat about raising kids there. The conversations may have covered where they went to school, their car, spouse, dog, or the medical problem they had last year, but it went further than the standard "How are you doing?"
They are your contacts, your sources for information. On the real organization chart, these are the people you go to when you need the unvarnished truth or something from their department. They are a back-channel way of getting things done quickly and with a minimal amount of hassle.
Some will be executives and managers but your contacts should come from all levels of the organization. [Secretaries in particular are very powerful.] These are the folks who may initially give you the "party line" but then, when you ask, "What's really happening here?" will shut the door and tell you as much of the truth as they can get away with, which may just be a hint, a subtle warning or a heads-up. In some cases, they may say that they cannot say anything and that, of course, says something. You'd do the same for them.
They are gold so long as you remember to put the person first and their role as a contact second. You should never do anything to embarrass them nor should you ask them to do anything unethical.
You also have an obligation to reciprocate. After all, you too are one of the Reliables.