What does downloading mean within Usenet?
It is similar to downloading content from a website, in the form of downloadable files that you can copy to your hard drive and store them for future use. Usenet downloads are a bit different, of course. Files are not really presented and grouped in the way they usually are on a website. Files in Usenet are commonly referred to as binaries. Binaries are file attachments to messages (articles) posted in Usenet.
Because of the inherent text structure of Usenet, files (binaries) cannot be posted there by themselves. They have to be inserted as a part of a text message. The text message does not have to contain any text, it may just contain the binary. Those binaries can be downloaded, of course. There are many newsreader applications, which download all binaries they encounter in a given newsgroup, or server automatically, when instructed to do so.
What Can You Download in Usenet?
You can find all kinds of binaries in Usenet. Depending on the server, or newsgroup you are in, you can find images, videos, games, animation, software applications of various kinds, programming code, and much more (it is impossible to list all kinds of files that are posted in Usenet). Binaries are usually posted in binary newsgroups but you can sometimes find text groups that have binary attachments posted in them. Often, binaries appear to be posted in newsgroups where they do not belong.
That happens because the posters (the individuals who post the messages) did not select the appropriate newsgroups where their material belongs and posted it in the wrong ones. Sometimes, messages are being posted in as many newsgroups as possible – this material is usually spam and is not tolerated by most Usenet providers. FastUsenet.org advises its users to report spam when they see it in their disclaimer: https://www.fastusenet.org/disclaimer.html