What do I do with .tar.gz or .tar or related files?

This is rather a common question.  The answers are as follows:-

To open a .tar file, issue the following command

tar xvf filename.tar

To open a .tar.gz file, issue the following command

tar xvfz filename.tar.gz

To open a .tgz file, issue the following command

tar xvfz filename.tgz

To open a plain .gz file, not a .tar.gz file, issue the following command

gunzip filename.gz