What do Sitecore layouts define?

Sitecore layouts are integral to the platform's content management framework as they define the page structure and how components are rendered on that page. This involves determining which components will be displayed, their arrangement, and how they interact with the content stored in Sitecore.

Each layout can contain one or more rendering zones that allow for the inclusion of various components, such as text, images, forms, or any other elements. The layout plays a critical role in delivering the front-end user experience, as it dictates how content appears to users when they access the site. It essentially bridges the gap between content and presentation, allowing developers and content authors to create dynamic web pages that react to users' needs.

In contrast, other options address different aspects of Sitecore. The back-end logic of an application involves custom code and server-side processing, while the security model pertains to user permissions and access control, and the database schema relates to how content is organized and stored in the database. None of these options focus on the presentation layer, which is where layouts are specifically designed to operate.