.bz2 is the filename extension associated with the bzip2 data compression format. It is an open source data compression and decompression tool developed by Julian Seward in 1996. The compression and decompression format is cross platform compatible and comes bundled with versions of Linux and UNIX. The latest of Bzip2 exists as an open source library for download and customization. Earlier versions have executables that can be run directly on Microsoft Windows.