A patch in Joomla! is a bunch of bug fixes which might or might not include fixes for security issues. The patch announcement will always tell you if a patch has fixed any security issues. A patch is a minor upgrade. For example, there is a patch to upgrade your website from Joomla! 1.0.0 to Joomla! 1.0.7. Patches are generally not available when bigger upgrades are involved. For example, there is no patch available for upgrading from Joomla! 1.0.14 to Joomla! 1.5.0. Before applying a patch you should always read the release notes and the upgrade instructions relating to the patch.