System users are those used to run services such as DNS, mail, web servers, and so forth. The reason for this is security; if all services ran as the superuser, they could act without restriction.
Examples of system users are daemon, operator, bind (for the Domain Name Service), news, and www.
nobody is the generic unprivileged system user. However, it is important to keep in mind that the more services that use nobody, the more files and processes that user will become associated with, and hence the more privileged that user becomes.