Zend Framework 3 is shipped with Twitter Bootstrap that is a CSS framework allowing for creating visual appealing and professionally looking web applications. It provides the base CSS rules, the simple layout grid, and useful interface components (like navigation bars, breadcrumbs, pagination, etc.)
In a typical web site, pages have common structure (for example, a typical page may have a navigation bar at the top, the body with page content, and the footer with the copyright information at the bottom). In Zend Framework 3, you define this common structure with a view template file called the layout. The layout template may have placeholder(s) in which ZF3 puts the content specific to a particular web page.
View helpers are (relatively) simple PHP classes that encapsulate a part of page rendering work. For example, they allow for composing the page of several parts, setting page title and meta tags, and creating the reusable widgets like navigation bar or breadcrumbs.