A RAR archive is a compressed file, or group of files.
A RAR archive could have a single small video, thousands of pictures, or 10 Gigabytes of files inside it. RAR's are very popular on Usenet because they let you compress sometimes large uploads in to the smallest package, but still keep them available to everyone because RAR decompression is found in every commonly used news reader (aka: Usenet software).
Though it allows compression like Zip files, it is different in two important ways. First, RAR files can be split in to multiple segments, sometimes even in to hundreds of segments. This makes them easier to handle. Second, you can repair them if they are damaged! This is critical for Usenet, because it would be infuriating to download a 5 Gigabyte file and find that you had all but 100 Kilobytes so the file was useless. Instead, your Usenet 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 Usenet 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 Usenet. Just know that these file endings aren't confusing, your news reader takes care of all the complicated parts, you just click and download!