Menus are a collection of links (menu items) used to navigate a website. The Menu module provides an interface to control and customize the powerful menu system that comes with Drupal. Menus are primarily displayed as a hierarchical list of links using Drupal's highly flexible blocks feature. Each menu automatically creates a block of the same name. By default, new menu items are placed inside a built-in menu labeled Navigation, but administrators can also create custom menus.
In Drupal 7, you can add, remove and rename menus and menu items/tabs. You can also configure a special block for a menu and you can specify the default menu to which new items will be added.
In many themes there are at least two menus, the Main (or primary) menu, and a secondary menu. The Main menu's links drive the main navigation structure for your site, and are often displayed prominently across the top or side of the site. The Secondary menu is often used for items like the copyright and privacy notices and are often displayed at the bottom or very top of the page in smaller text. You can specify which menus are used for the main and secondary links.
When adding or editing site content, you can also specify the contents' menu item. You can specify a default menu for the content authoring form.