From Joomla! Documentation

In Joomla!, a Menu is a set of menu items used for website navigation. Each menu item defines an URL to a page on your site, and settings that control the contents(articles, category(ies) lists, tagged items, etc.) and style(module(s), layout) of that page. Additionally, each menu has a Menu Type, these are shown in the menu manager, see Adding a new menu. The menu type will actually be the Unique Name or alias that will be used to create human-readable URLs if Search Engine Friendly URLs are turned on. Menu items can have any number of subitems.