compress and uncompress:
  gzip, gunzip, gzcat (or zcat), gzmore (or zmore), zgrep

             gzip
  somefile --------> somefile.gz
          <------- 
            gunzip

    gunzip === gzip -d   (decompress)
    gzcat  === gzip -cd  (decompress and stdout)
    zmore  === gzip -cd | more
    zgrep  === gzip -cd | grep

  bzip2, bunzip2, bzcat, bzmore, bzgrep : similar as gzip's stuff

            bzip2
  somefile --------> somefile.bz2
          <------- 
           bunzip2


tar: tape archive, combining several files together for archiving, 
  or extracting some file from a backup tar file

  tar cf new-created.tar file-a dir-b   # create a new tar file
  tar xf some.tar                       # extract contents 
  tar rf existed.tar file-c file-d      # append files to an existed tar file
  tar tf existed.tar                    # show the contents of a tar file

  The option 'v' can be used to give more verbose information.

  tar cvf new-created.tar .......
  tar xvf some.tar
  tar rvf existed.tar .....
  tar tvf existed.tar  

  The file '-' is treated as stdin or stdout.

  tar cf - file-a file-b ..   # the created tar file is sent to stdout
  tar xf -                    # the tar file is read from stdin

  Often two tar commands are combined with a pipe to make a copy of something.
  For example, the following command make a copy of filea and dira under
  another-dir.

  tar cf - filea dira | (cd another-dir; tar xf -)
  
  Even the source and destination can be at different machines.

  ssh remote-machine 'tar cf - .......' | tar xf -    # note the quote

  A tar file is often compressed. For example: 
    lame-398-2.tar.gz       gzipped lame-398-2.tar
    scheme48-1.8.tgz        tgz means tar.gz
    ypserv-2.22.tar.bz2     compressed with bzip2

  You can create a compressed tar file either by
    $ tar cf new.tar fa fb ....
    $ gzip new.tar
  or
    $ tar zcf new.tar.gz fa fb ....        z: gzip
  or
    $ tar jcf new.tar.bz2 fa fb ...        j: bzip2
    
  A compressed tar file can be extracted either by
    $ gunzip old.tar.gz                 # first  step, uncompress
    $ tar xvf old.tar fa fb ....        # second step, untar
  or
    $ gzip -cd old.tar.gz | tar xvf -   # two steps combined with a pipe
  or                                    # GNU-tar can do two steps together
    $ tar zxvf old.tar.gz fa fb ....    z: gzip
  or
    $ tar jxvf old.tar.bz2 fa fb ...    j: bzip2