The newsgroups file lists groups along with their descriptions. Not all groups on the server have to be listed, and not all groups in the file have to exist on the server. The file is meant purely as information to the user.
The format is quite simple; a group name, a tab, and the description. Here's the definition:
newsgroups = *line line = group tab description <NEWLINE> group = <non-white-space string> tab = <TAB> description = <string>