The component class of a page is the central (or main) content of page. For example paragraphs containing text.
Everything surrounding that central part is created by the so called "master" which consists of a Master Template and Master styles.
The Master Template can embed so called "boxes" - which are ordinary components created by a box generator. That allows using any component as box.
Location & Inheritance
The Master Template is inherited for all child pages, thus normally the Master Template is can be found at the Root Component.