Think of it like Pirate Bay, but for newsgroups. Newsgroups are massive with some providers archiving binary groups for up to 4 years (and growing). Sites like NZBMatrix index the content and provide nzb files which tell your client what to download so that you don't have to constantly download the headers for a bunch of sites and search through them. Sites like NZBMatrix were also somewhat curated to help keep passworded files out of the results.