The document discusses the theory of plate tectonics, including what plates are, how they move, and the three types of plate boundaries. The three types of boundaries are divergent boundaries, where plates move apart; convergent boundaries, where plates move towards each other; and transform boundaries, where plates move past each other laterally. Each boundary type results in different geologic features and events due to the stresses caused by the ways plates are pulled, pushed, or sheared at their edges.