TGZ is the file extension for a so-called tarball archive, which was compressed with gzip. Gzip stands for GNU zip, which is a free compression method based on the Deflate algorithm. Since Gzip archives can only contain a single file, the files to be archived are first combined with tar to form a tarball and then packed with gzip. These files have either the double ending .Tar.Gz or simply .Tgz.
TGZ archives can be unpacked by just about any packer, be it Linux, Windows or Mac