What is a RAR File?
Simply put, a RAR file is a compressed file or a group of files. A RAR file could have a single small video, thousands of pictures, or 10 Gigabytes of files inside of it.
Though it performs file compression like the Zip format, it is different in two important ways. First, RAR files can be split into multiple segments, sometimes even into hundreds of segments. This makes them easier to handle because you can download one segment at a time and not have to start over if your download fails. Second, you can repair RAR files if they are damaged!
Why Usenet has so many RAR files.
RAR's are very popular on Usenet because they let posters compress large uploads into the smallest package, but the RAR format still keeps the upload available to everyone because RAR decompression is found in all commonly used newsgroup readers.
Having the ability to repair damaged files, or to patch missing segments, is critical for Usenet. With very large files, it would be infuriating to download a 5 Gigabyte file and find that you had successfully downloaded all but 100 Kilobytes so the file was useless. Instead, your news reader software just sees that 100 Kilobytes are missing, finds the PAR (repair) files for the post and does some fairly complicated math in the background while you watch the progress bar hang for a couple of seconds. Then your file is as complete as when the poster uploaded it! In fact, it's a perfect copy. We told you it was complicated math!
So when you mix these two advantages, you get a system that is ideal for Usenet. Large files can be posted to a newsgroup, multiple random segments can be missing, and your software can download a perfect copy without you having to do anything.
As you would expect, this is the preferred method for handling all but the smallest files posted to binary newsgroups. Just know that these file endings aren't confusing, your news reader takes care of all the complicated parts, you just have to click and download!