tar The .tar filename was originally designed to be used as a TApe ARchiver to store entire file systems and files as single files on tape storage medium. It was designed by AT&T in 1979. Today, the format more commonly refers to an archive or collation of multiple files into a single file with the .tar filename extension. The format is an open source file archive format commonly associated with the UNIX tar command however other data compression tools are capable of effectively compressing and/or decompressing files saved with the .tar file format. Doing so typically adds the compression filename extension to the archive file for example .tar.bz2.
t7z The .7z filename extension is associated with the 7z compressed file archive format and the open source 7-zip compression utility both developed by Igor Pavlov. The format had its initial release in 1999. It consists of a start header 32 bytes in size which contains the signature and link to the ending header, followed by the compressed data, a metadata block, and finally the end header. 7z supports limited recovery options for 7z archive files which can open but for one reason or another cannot extract due to CRC or other data related errors.