REV files are Recovery volume files that have been created by RAR file compression and archiving software. REV files contain redundancy data that can be used to reconstruct missing files within a RAR volume set. REV files are only used with multi-volume archives. The number of REV files present cannot be less than the number of corrupt or missing volumes.
RAR (pronounced Rare or rar) is a compressed archive format developed by Eugene Roshal. His main product is WinRAR, which is shareware.