The Customize API is object-oriented. There are four main types of Customizer objects: Panels, Sections, Settings, and Controls. Settings associate UI elements (controls) with settings that are saved in the database. Sections are UI containers for controls, to improve their organization. Panels are containers for sections, allowing multiple sections to be grouped together.
Each Customizer object is represented by a PHP class, and all of the objects are managed by the Customize Manager object, WP_Customize_Manager.