There are three main types of volcanoes classified based on their activity and structure: 1) Active volcanoes erupt regularly, dormant volcanoes have erupted in the past but are currently quiet, and extinct volcanoes are unlikely to erupt again due to lack of lava supply. 2) Shield volcanoes have broad, gentle slopes built by fluid lava flows. Composite volcanoes have a steep, conical shape formed from layers of hardened lava and ash. 3) Examples include Mauna Loa, a shield volcano in Hawaii, and Mount St. Helens, a composite volcano in Washington, USA.