For merchants, the catalog represents their product inventory. Within Magento’s architecture, the catalog consists of categories, products, and product attributes.

Each Magento store has only one product catalog, which is shared by all store views. In a multi-store installation, each store can have a separate catalog, or share the same catalog. The current store catalog is determined by the default root category, visible to the user in the top navigation (main menu) of the store. (The term "root category" may be confusing, because the "root category" isn't really a category at all, but a container for the menu, which consists of categories and sub-categories.)

You can create as many root categories as you want, but only one (the default) can be used at a time.