A volcano is a place where the earth's crust is weak, and seismic activity leads to expulsions of lava, gases and brimstone during eruptions. Volcanoes are usually situated either at the boundaries between tectonic plates or over hot spots. Volcanoes may be either dormant (having no activity) or active (near constant expulsion and occasional eruptions), and change state unpredictably.
Volcanoes on land often take the form of flat cones, as the expulsions build up over the years. Under water, volcanoes often form rather steep pillars and in due time break the ocean surface in new islands.
Science has not yet been able to predict with absolute certainty when a volcanic eruption will take place, but significant progress in judging when one is probable has been made in recent time.
Public domain picture from NASAof Tambora, Indonesia.