When Magento is installed, a hierarchy that includes a main website, store, and store view is created. You can create additional websites, stores, and store views, as needed. For example, in addition to your main website, you might have additional websites with a different domain. Within each website, you can have multiple stores, and within each store, separate store views. Many installations have one website and one store, but with multiple store views to support different languages.
Before you begin, plan your store hierarchy in advance because it is referred to throughout the configuration. Each store can have a separate root category, which makes it possible to have an entirely different set of main menu options for each store.