This is a non-FreeBSD specific term. It features in this wiki for reference.
A 'service' is a general term used to refer to a background process running on an operating system, usually associated with Microsoft operating systems based on NT (including later incarnations Windows 2000, XP, 2003 and Vista, both workstation and server). On UNIX and Unix-like operating systems such background processes are called daemons.
The services on Windows platforms are controlled through a Control Panel applet. On Windows 2000 and later this is launched as an MMC snapin called 'services.msc'. Services can be stopped, started and paused and can be set to start automatically, manually or simply be disabled.
Typical services deployed on a Windows 2003 Server (shipped with the operating system CD-ROM itself) include DHCP, DNS and IIS (Internet Information Server) but others exist separately for handling email, backups, etc.