If you used the option "z" before (while packing), you have to use it again while unpacking because "z" means -- use gzip compression. In that case you should use extension .tgz resp. .tar.gz not .tar nex time.
Finally, you can use either
Code:
tar xvf domains.tar
or
Code:
tar xzvf domains.tgz
depending on if you used the "z" while packing.