The services tag wiki excerpt states:
Services are processes running regardless of users logging in. They provide access and features to different programs or processes
The terminology is clearly Windows only, the Unix equivalent is called daemons.
There's a different kind of Service on OS X though, the kind referred to in the questions tagged osx+services and some of those tagged services+mac. These are commands available in certain or all programs that may receive some type of input, like selected files or text, and can produce output. This website shows a few examples.
They can be provided by the OS, applications, or created by users using automator. Quite a few questions on Super User are about these.
Do we need to broaden the scope on the tag wiki?