Originally Posted by fris

RAR stands for Roshal ARchive. It is a non-documented archive file format that supports data compression, error recovery, and file spanning.[2] It was developed by a Russian software engineer, Eugene Roshal (the first letter of his surname contributing to the name of the archive format), and is currently licensed by win.rar GmbH.[3]

The filename extension used by RAR is .rar for the data volume set and .rev for the recovery volume set. In previous versions, if a RAR-archive was broken into many smaller files (a "multi-volume archive"), then the smaller files used the extensions .rar, .r00, .r01, .r02 etc.