To begin with, for most people a standalone drupal doesnt give you much option to do, however its plugins/modules from the drupal community can expand that functionality into a much wider scope. Your own version of modified theme can also be installed to change how drupal essentially looks.

Perhaps, you are wondering why we should need such work, that's simply because once you upgraded your site to a better version of drupal (let's say from 6.6 to 6.7) then the Drupal 'Upgrade.txt' instructions specifically say that we should backup our 'sites' folder since all other folders will be overwritten during the upgrade.

So, do not ever put your contributed modules/themes outside 'sites' folder otherwise you may have lost it during the upgrade. Well, some of you may comment, sometimes we do need to alter the core drupal module to change how forms looks and such.

I would say no, you can alter how form looks from your own defined theme (template.php) thanks to Drupal 6 theme registry. You would only need to hack the drupal core module if you are required to change form submission action or add new validation rules. I wouldn't go in too much details as I'll explain it in-depth on my next post .