Volcanoes are found through out the solar system and are vents in the surface of the planet. Magma is fed up into the volcano and stored at high pressure inside a magma chamber. As the pressure reaches maximum and there is no more room for the magma to expand and pusses up the throat. It brakes the top causing ash, lava*, and rock to cover the surrounding area.The different kinds of volcanoes are based off of the levels of silicon in the lava. The types of lava are mafic (oozes), felsic (giant explosion!!), intermediate (kabooms), and ultramafic lava (flows). These are one of the ways to classify volcanoes.
Mafic lava contains less than 52% to greater than 45% silica and slowly creeps along the ground. It forms in settled volcanoes, fissure eruptic volcanoes, cinder cone volcanoes, and strato volcanoes. Shield Volcanoes are formed by lava spreading out from the cinder in a flat disc and slowly building up. Fissure eruptic volcanoes are like mini-volcanoes, lava flows down the side and shoots about eight feet into the air but does little damage. Cinder cones are a type of volcano that is only about 1000 feet tall (as high as a hill). They ooze lava and most have a crater on top. Stratovolcanoes are mainly found under water and form on subduction zones. It has a thicker wall than most volcanoes allowing it to build up more pressure causing a larger explosion.
Felsic lava has greater than 63% silica and is formed in stratovolcanoes and caldera. It explodes out of the top of the volcanoes and are one of the most destructive. Caldera are large magma cambers that have vents lidding up to the surface, when the presser reaches max it erupts. They have been detected on Io, Venus, Mars, and even the Moon; an example of one on Earth is Yellow Stone.
The last two types, intermediate and ultramafic, are not very common. Intermediate lava has less than 63% silica but greater than 52% silica. It does not explode but flows slowly. It forms in andesite volcanoes. Andesite volcanoes form when two tectonic plates rub against each other and get hot enough to form magma. Few eruptions have yielded ultramafic lava since the Proterozoic period. It has less than 45% silica. It is the hottest magma and is more fluid than Mafic, and is (like its name says) ultra rare. It formed in all the same volcanoes that mafic magma does but at a time when the earth was a lot hotter.
Volcanoes have changed life as we know it many times. They hurt us and help us. Hawaii has sheltered hundreds but Krakatoa killed hundreds. They provide heat and fertile soil but thousands have died trying to get to it. But, in the end we would not be here without them.
* When magma brakes the surface it is called lava.