By system accounts I mean the ones that comes preinstalled with the system, for example nobody, daemon and bin. I know these tend to vary depending to the distro and the installed packages, but:
is there a standard set of system accounts?
What are the security implications behind these accounts (i.e. why run something as user nobody)?
Where do I read about it?
There isn't much specified anywhere. You can read a little bit at the Linux Standard Base Core Spec and specifically about users here.
The general reason for different accounts is a basic security premise. Restrict functions to a single user only, so that user can't interact with unrelated stuff. You don't want a user to be able to reboot the server just because you used the same user ID / group for starting and stopping the system as you did for managing print queues.
