Tar stands for tape archive it’s a old unix way of collecting lots of files into a single file for transport or storage on a tape drive. Later authors have combined gzip with tar for compress on the fly.
So to open a tar.gz use the following command:
tar xvfz filename.tar.gz
To open a tar do this
tar xvf filename.tar
To create a tar file
tar cvf filename.tar files_or_location_to_add.tar
To create a tar.gz
tar czvf filename.tar.gz files_or_location_to_add.tar.gz
More information on tar can be found by
man tar