Usenet (NNTP) is a protocol
somewhat similar to email. Unlike email, the default communication style is in "groups" (sort of like a mailing list for a specific topic). One posts individual messages (like one email) to a group; replies are threaded, much like mailing list discussions. Historically the groups were organized somewhat hierarchically.
The end result, with a good news reader program, is somewhat like a large global collection of topic-specific forums.
Binary posts are just a bunch of posts to the same group with a message Subject matching some pattern (usually with ascending numbers counting the posts). The content is encoded to work around the limitations of the protocol and/or provider. Historically this was "uuencode", but you can also do base64 (like email attachments) and the new jazz (hah! New in 2003, anyway) is "yEnc." yEnc relies on 8-bit safe NNTP servers and only encodes the few characters not permissible in NNTP messages (periods at the beginning of lines, I forget the rest). So you get much better encode ratios (~101-102% of original size) than with something like base64 (133% of original size).
NNTP providers, especially in binary groups, are subject to occasional message loss. So posters will typically generate Reed-Solomon forward-error-correction blocks using a program called "par2." This allows one to download binaries that are missing some messages, as well as some additional FEC (potentially on an as-needed basis), and rebuild the original content. Generating and recovering from par2 files is extremely computationally expensive.