Wikipedia
Parchive (a portmanteau of parity archive, and formally known as Parity Volume Set Specification) is an erasure code system that produces par files for checksum verification of data integrity, with the capability to perform data recovery operations that can repair or regenerate corrupted or missing data.
Parchive was originally written to solve the problem of reliable file sharing on Usenet, but it is now commonly used for protecting any kind of data from data corruption, disc rot, bit rot, and accidental or malicious damage. Despite the name, Parchive uses more advanced techniques that do not utilize simplistic parity methods of error detection and correction.
As of 2014, PAR1 is obsolete, PAR2 is mature for widespread use, and PAR3 is an experimental version being developed by MultiPar author Yutaka Sawada. The original SourceForge Parchive project has been inactive since November 9, 2010.