求翻译：Newsgroups, unlike mailing lists, use a special network of computers called the Usenet. Each of these computers maintains the newsgroups listing. There are over 10,000 different newsgroups organized into major topic areas that are further subdivided into subtopics. Contributions to a particular newsgroup are sent to one of the computers on the Usenet. This computer saves the messages on its system and periodically shares all its recent messages with the other computers on the Usenet. Unlike mailing lists, a copy of each message is not sent to each member of a list. Rather, interested individuals check contributions to a particular newsgroup, reading only those of interest. There are thousands of newsgroups covering a wide variety of topic areas.是什么意思？

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