Plate Boundary Theory explains how the Earth's outer shell, or lithosphere, is divided into large sections called tectonic plates. These plates float on the semi-fluid layer beneath them, known as the asthenosphere. The interactions between these plates at their boundaries can lead to various geological events, such as earthquakes, volcanic eruptions, and the formation of mountain ranges. There are three main types of plate boundaries: divergent, where plates move apart; convergent, where plates collide; and transform, where plates slide past each other. Understanding these boundaries helps scientists predict geological activity and comprehend the Earth's dynamic nature.