Big RAR files are splitted (usually for fitting on a floppy or for uploading to websites with upload size limitations like rapidshare) into small parts.
Suppose you have a rar file file.rar with a size of 500 MB and want to upload on some websites like rapidshare. So you'll split it up to 10 files with 50 MB each. So now the file name will be like
file.r01
file.r02
.... file.r10
which can be extracted by extracting the first file in the series and placing all other parts in the same directory.