A file with the extension .DAR is an archive file created using 'DiskArchive'(DAR). An archive file is usually a group of files compressed into one file.

Compressed .DAR file(s) or archive(s) are files that are reduced in size either for storage or for faster transmission via the internet, etc.

DiskArchive is often used for creating 'split archive' .DAR files, a split archive is an archive that has been divided into several or numerous parts, the benefits of this being that by splitting a single archive into numerous parts, you can store the archive in chunks across multiple disks.

DAR is a 'Linux' based utilty, with Linux being a free open source developed operating system.