A UseNet client application that implements the NNTP Protocol and accompanying message format, much like a MailClient implements SMTP and accompanying message format and a WebBrowser implements HTTP and HTML, JavaScript, etc.

Integrated NewsReaders are included with the Netscape/Mozilla browsers and OutlookExpress (not MicrosoftOutlook); though using the latter might make you unpopular due to a large array of "perks" and annoyances that ignore established Netiquette.

A list of popular standalone NewsReaders for Linux includes:

With GUI:

For the console: