Files that gives you control over the display of modules or components.

Override files are placed in special folders and can be used in place of the standard layout files. Unlike Template Overrides, which override layout files of the same name, Layout Overrides are additional layout files that give you more options and control.

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