this is an example of a tar command i use on my linux server all the time:

tar -pczf www-2008-oct-13.tar.gz www/

lets say i want to compress the /var/www directory this is what i would do, i would cd to the /var directory:
cd /var

then i can compress the www/ direcotry and im going to call it backup.tar.gz

tar -pczf backup.tar.gz www/

how how do i untar it? once i have tar it, i can untar it with this command:

tar xvfz backup.tar.gz

hope this helps