A service is, in computer science terms, a functionality (of a software or of a device), that hides its implementation details to the user. To be able to use the service, the service has to declare what the service does at its interface. In the old days, you would get a manual, in the new days you would descriptions in some fancy service description language.
Besides discussions how to write down what a service does, I feel there should also be some thoughts about what all should be described about a service. It’s quite easy to miss something important as the following video from our new building shows.