Plugin Development

Core hacks are bad, a way around core hacks is to code a plugin or do an override. Plugins replace content or add content or modify content. For instante we have a recaptcha plugin, enabling it and configuring it can put a recaptcha on your contact page. As a core hack, you would have to modify components/com_contact/somefiles here... So if a new version of joomla happens to upgrade that area your core hack just broke. As a plugin it will never break.

Tutorial:Plugins - Joomla! Documentation - http://docs.joomla.org/Tutorial:Plugins
Screen.plugins.15 - Joomla! Documentation - http://docs.joomla.org/Screen.plugins.15