In this lecture, the different classes of volcanoes are described: Shield, Cinder Cone & Stratovolcanoes. We explain the difference between magma and lava, and the reasons why the nature of volcanic eruptions are determined by the viscosity of the magma. Several examples of active volcanoes around the world, including Mt. St. Helens, Mauna Loa, Kilauea and Mt. Vesuvius are studied.