In 1969, it started out as a single network for research sponsored by ARPA, the American Department of Defence Advanced Research Projects Agency. The Internet carries many kinds of traffic. Technical details are necessarily simplified. The first element usually refers to a specific machine. In principle, one can connect from any machine to any other.
Special gateways allow some access to machines on other kinds of networks.
This program allows a user to transfer files to and from a remote network site. Normally you need access privileges, but some services, e.g. library or educational database lookups are provided freely without restrictions. Thousands of anonymous FTP servers exist. A lot of research institutions make available research papers and software that way. Those news groups are organised in hierarchies.