Whilst the layout of the storefront is dictated by the theme, the displayed content is managed through blocks. This includes navigation (such as header and footer menus), promotional imagery, static text and HTML snippets. Each block can contain multiple items of different types, allowing for features like image carousels and mega menus.
For more information about creating and editing blocks please see "What is a block?".
For more information about using blocks in your storefront please see "How do I display content blocks on the storefront?".