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.
txz The TXZ file extension is typically found in archive files which are created using a combination of TAR and the XZ utilities. TXZ files contain multiple files which are first archived using Tar and later compressed using the technology of XZ compression. TXZ files are known to be smaller in comparison to TGZ files. From a more basic perspective, XZ utilities compress TAR archive files just like how GZ utilities compress various TAR archives, the only difference being in the compression ratios and the resulting file sizes after compression.