You forget one "port-number"
A service, is a service a server can offer.
F.e. ftp
ftp is a service taken in /etc/services,
it has a specific port onwhere it should communicate with.
Therefor a daemon (inetd) is started that keeps track of any incomming or outgoung requests on that port.
This daemon runs as a process as it's claiming memory/cpu-time/IO-resources.
Subsystem is quiet unknown with me, Maybe you mean the child of a Parent process. Sometimes a process can fork and create a child to have a job done for him. We call this the parents child.
f.e. :
running a shell gives you a process (ps -ef | grep sh | grep -v grep). Now performing an "bdf ." will fork your shell into a process that will find out the diskusage for your current directory. when it is finished the child automaticly dies (stops) and goes back to it's parent.
Well, enough fast-course Unix
Anymore questions, just let us know.
Regs David