Components are a section or subsection of a designed space. They take up a significant portion of the screen, and may be as large as the viewport (or, depending on your point of view, larger).
Components are concerned with displaying large amounts of information, or allowing the user to interact with the system is some significant, primary manner. Combining them, with the small, reusable, interactive or display Widgets (see that section) gives an unlimited number of options for design.