If your taring the htdocs folder, then there is a sub folder or file within the htdocs folder that is being written to by another program on the server and therefore is changing while tar is trying to compress the htdocs directory.

tar --help

/var/www/localhost/htdocs <-- typical path

cd /var/www/localhost

tar -cjf htdocs.tar.bz2 htdocs/

That will tar the entire htdocs directory and everything in it.

As i said, if there is a file being wrote to while its taring then its going to spit out the error again and say something changed while reading it. You will need to use the exclude switch in the tar command

You are creating a backup to a file and you place that file in the directory that your are archiving. The file that is "changing" is the backup file itself.