Be forewarned, I am no longer as academic as I am practical. Just be thankful you can't hear me mumble. 2.0.27 are used. A constant running background process. In addition these processes are unattached from any specific user or device.
Examples are lpd and kerneld.
You may need them as a architecture for programs that gathering data from a device, or memory compression, or print queuing. All of which don't need a UI or a user associated. If your daemon is critical to your system, often there will be an entry in the init hierarchy. If your daemon is really important, then study how other daemons are spawned. If the daemon is forked.