To extract an uncompressed archive: 
tar -xvf /path/to/foo.tar

To extract a .gz archive:
tar -xzvf /path/to/foo.tgz

To create a .gz archive:
tar -czvf /path/to/foo.tgz /path/to/foo/

To extract a .bz2 archive:
tar -xjvf /path/to/foo.tgz

To create a .bz2 archive:
tar -cjvf /path/to/foo.tgz /path/to/foo/