The .rar format is the output of an archival system that splits up large files into many smaller, compressed files. Posts in the multimedia groups usually are in this format and it is sometimes used in the MP3 groups. A shareware program called WinRAR is used to re-combine all the parts once you download them.
When you find a group of .rar files you are interested in, make sure that all the .r0* files exist before you start downloading them. You will need the .rar file and the .r00 through .rXX files to decompress the entire archive. If a RAR archive is split into more than 100 chunks, the file extension continues to .s00 through .s99 and then .t00 through .t99. Each piece of a .rar series will all be the exact same size except the .rar file itself, and the last one in the series, which will be smaller. So if you find a 15 part RAR archive, you will see a .rar, the .r00 through .r14 all being the same size, and a .r15 which will be smaller (or possibly the same size if it happened to be an even multiple of the file size but this is rare).
To combine all the pieces of a RAR archive, you need to download and install a shareware program such as WinRAR. They offer a free trial just like NewsBin does.
Use the "Find In Subject" feature to isolate the .rXX files using Regular Expressions (RegEx). This RegEx will show all the .r?? files: \.[rst][a0-9][r0-9]. In english, this means look for a "." (the "\" escapes the ".", otherwise it means "match any single character") then match an "r", "s", or "t", then match either an "a" or a digit 0 through 9, then match an "r" or a digit 0 through 9. If you are looking for a specific series like "amovie.rXX", use amovie\.r[a0-9][r0-9].
Here is an example of NewsBin showing a posted .rar file from alt.binaries.movies:
Notice that all the files are 15MB except for the .rar which is 14.91MB and the last file in the series, .r30, which is 13.51MB. This is a complete post because all the files from .r00 through .r30 including the .rar exist.
Once you download all the .rar files, go to the "Downloaded Files" tab and double-click on the .rar file. This will automatically launch WinRAR which will combine the files for you (assuming you have installed WinRAR as we suggest above) into one big file. The resulting file can then be played with Windows MediaPlayer, RealAudio, or whatever player you have installed for the resulting file format.