A "service" in windows IS in fact what Duffy described.
In linux it's a "daemon".
Enterprise Service Component (ESC) is another derivative which does bigger and bette stuff again.
Now, a Service in windows is a dll, and can be loaded directly into the Global Assembly Cache (GAC) then accessed from here at runtime and instantiated in a client application, or started at boot time.
Just thought an FYI may have been in order