An NZB file is an index (or "list") of newsgroup articles. You can directly import an NZB file into some newsreaders, such as Android Newsgroup Downloader. NZB files were invented by the site www.newzbin.com, but are now popular and created by many other sites.
The reason for using an NZB file is to avoid having to download large lists of headers. Many of the binary newsgroups contains millions of headers, and downloading all of the headers in order to search them can consume a lot of time. Thus, sites like newzbin allow you to search the headers and then download an NZB file that contains the search results. The NZB file is much, much smaller than the number of headers in the newsgroup, and contains only the headers you are interested in. You can then import the results directly into your newsreader.
It's also becoming popular for people to post NZB files along with the content that they're posting. For example, a person my post a series of RAR files, PAR2 files, and then a NZB that lists all of these files. That way, a downloader with a NZB-compliant downloader can download and import the NZB file and get exactly what he's looking for.
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