Template style is a feature introduced in Joomla 2.5 that allows users to assign different template styles to individual menu items. By default, Joomla! assigns a template style to all menu items upon installation. A yellow star indicates the default template style in use. A default template style can be partially or completely overridden by assigning different template styles to the desired menu items in order to obtain a different look for their respective pages.

A template style can be assigned to menu items one of two ways.

Template manager Extensions ? Template Manager

Editing a menu item under Menus ? Menu Name ? Menu item

Reference: http://docs.joomla.org/Glossary