A volcano is an opening or rupture in the crust of the Earth or the planetary mass object which allows the hot lava, volcano ash and gases to escape from below the surface. On Earth volcanoes are generally found where tectonic plates are diverging or converging. Erupting volcanoes can pose many hazards. Volcanic ash can be a threat to aircraft, especially with jet engines where these particles are melted by a high operating temperature. Large eruptions can affect the temperature as the ash can cover the Sun. Historically volcanic winters have produced catastrophic famines.

Some volcanoes are active which either erupt continuously or continually,
while some are passive or sleepy, they erupt after long time,
while some are extinct, it means they are unlikely to erupt in future.

The most common look of a volcano is conical mountain throwing lava, poisonous gases, ash,  from a crater at its summit, otherwise its features are much more complicated.

Volcanoes are found not only on land but also in the sea or ocean.  When they are on shallow sea floor, they blast steam and rocky debris high above the surface of the sea, while many others lie at such great depths that the tremendous weight of the water above them prevents the explosive release of steam and gases from coming above the earth surface. Sometimes they may become so large that they break the ocean surface as new islands.

Hawaii and Indonesia, are the biggest places for volcanoes.

Some Continuous Erupting Volcanoes
Keelauea, the famous Hawaiian volcano, has been in continuous, effusive eruption for 30 years, and has the longest-observed lava lake
Mount Etna and nearby Stromboli, two Mediterranean volcanoes, are in "almost continuous eruption" since antiquity
Mount Yasur, in Vanuatu, again in Hawaii, has been erupting "nearly continuously" for over 800 years