From Joomla! Documentation
No Joomla!, um Menu é um conjunto de itens de menu utilizado para navegação no "site" da Web. Cada item de menu define um URL para uma página no seu "site", e mantém as definições que controlam o conteúdo (artigos, listas de categoria(s), itens etiquetados, etc.) e o estilo (módulo(s), layout) dessa página.
Creating menus is rather simple. From the Admin Menu, in the backend of your Joomla! site, you choose: Menus > Menu Manager > Add New Menu. On the opened to you Menu Manager: Add Menu page enter your menu title into the "Title *" field (mandatory), your menu type into the "Menu type *" field (mandatory) and, if you wish, your menu brief description into the "Description" field (optional). After that you can start adding new menu items to this new menu.
Qualquer "site" da Web pode ter mais do que um menu.
A Menu isn't automatically shown on the site's front-end. You need to create a Menu Module using the Module Manager and instruct the module which Menu to show. Subsequently, you can select where the module should appear on the site, by choosing one of the available Module Positions of the active Template.
There is even more control on when the menu should appear, by adjusting the settings of the menu module. So, you can set it to appear in all or just a few pages of your choice (assigning the module on menu items), or make it visible to all, or only specific user-groups (e.g. registered users).
It is also possible to create Split menus.
A Menu that isn't shown by any module is commonly called a hidden menu. Hidden menus can be used to create URLs that are not visible on any page, within menus. For example, this can be very useful, if we want to create a single article page, with a custom URL, and specific content settings (layout, modules), that could be accessible through another page (article, blog category, module, etc), but we don't want to display a Menu Item for it, anywhere on our Menus.
There is also a "Display in Menu" setting on the Link Type tab within each menu item that can be set to No, which will hide an individual menu item from being visible on the front end.