NNTP is the Network News Transport Protocol used to transfer Usenet news around. Notice that questions about the usage of news clients are considered off-topic.
The Network News Transfer Protocol (NNTP) is an application protocol used for transporting Usenet news articles (netnews) between news servers and for reading and posting articles by end user client applications.
The messages and protocol are defined in RFCs from The Internet Engineering Task Force (IETF). The messages are defined in RFC 5536 and the way these are transferred is defined in RFC 5537.
Users read messages using a client which connects to a server. Servers pass messages between each other so that all messages in the group that the server manages are available to the client. Or, as the netnews RFC 5537 say in 1.1, "Basic Concepts":
"Netnews" is a set of protocols for generating, storing, and retrieving news "articles" whose format is defined in [RFC5536], and for exchanging them amongst a readership that is potentially widely distributed. It is organized around "newsgroups", with the expectation that each reader will be able to see all articles posted to each newsgroup in which he participates. These protocols most commonly use a flooding algorithm that propagates copies throughout a network of participating servers. Typically, only one copy is stored per server, and each server makes it available on demand to readers able to access that server.
"Usenet" is a particular worldwide, publicly accessible network based on the Netnews protocols. It is only one such possible network; there are deployments of the Netnews protocols other than Usenet (such as ones internal to particular organizations). This document discusses the more general Netnews architecture and protocols.
Clients tend to be applications that run on users' machines although there are some gateways to Usenet including Google Groups.
