Usenet is a system that allows you to post and receive messages as well as files. Every file or message is saved for a certain amount of time. Thus Usenet can be compared to a forum. Every message or file is uploaded to a newsserver where other users can log on and retrieve them. Just like a certain category in a forum the data is sorted by categories called newsgroups. If you are interested in files so-called binary newsgroups are the ones you should look out for (e.g. alt.binaries.software). There are a lot of newsgroups to probably every topic imaginable. Due to the huge amount of data there is a retention time determining how long files are made available.
To get access to Usenet you need a piece of software, which is called a newsreader. Especially if you would like to download files a newsreader is indispensable. Back in the days Usenet was a playground for geeks. Since Usenet uses a lot of appointed servers containing identical data it offers immense download rates.
However, it used to be quite tricky to eventually merge the individual files. Yet with a reliable newsreader downloading and merging files from Usenet is a piece of cake.