Short answers, pulled from the story.
A volcano is a vent or fissure in the crust of a planetary-mass object that lets hot lava, volcanic ash, and gases escape from a magma chamber below the surface. On Earth they form mainly along tectonic plate boundaries where plates diverge or converge, and also at hotspots fed by mantle plumes rising from the core-mantle boundary about 3000 km deep.
The main types include shield volcanoes with broad profiles built from low-viscosity basaltic lava, stratovolcanoes or composite volcanoes built from alternating lava and tephra layers, lava domes formed by slow viscous eruptions, and cinder cones piled from scoria and pyroclastics. Other types include mud volcanoes and cryovolcanoes that erupt ice on moons of Jupiter, Saturn, and Neptune.
Active volcanoes have a history of volcanism and are likely to erupt again, while extinct volcanoes are considered unlikely to erupt because they no longer have a magma supply. Dormant volcanoes have not erupted in a long time, generally since the start of the Holocene about 12,000 years ago, but may erupt again and are technically still geologically active.
A supervolcano is defined as a volcano that has produced one or more eruptions of over 1000 km3 of volcanic deposits in a single explosive event. They are very rare, with four known from the last million years and about 60 historical VEI 8 eruptions identified in the geologic record. Known examples include Yellowstone Caldera, Lake Taupō in New Zealand, and Lake Toba in Sumatra.
Large eruptions send ash and sulfuric acid droplets into the atmosphere that obscure the Sun and cool Earth's troposphere, and have historically caused volcanic winters and famines. The 1815 eruption of Mount Tambora produced the Year Without a Summer, and the Lake Toba supereruption about 70,000 years ago may have created a population bottleneck affecting all humans today.
Yes, volcanoes are very numerous on Venus, and Mars hosts vast extinct shield volcanoes including Olympus Mons, the largest known volcano in the Solar System. Jupiter's moon Io is the most volcanically active object in the Solar System, while cryovolcanoes erupting ice have been observed on Triton, Enceladus, and Titan.
The word volcano originates from the early 17th century, derived from the Italian name Vulcano, a volcanic island in the Aeolian Islands of Italy. That name in turn comes from the Latin name referring to Vulcan, the god of fire in Roman mythology.