A volcano is a mountain. Rock from the interior of the earth comes to the surface from it. The rock is so hot that it is molten. The name "volcano" comes from the Roman god of fire, Vulcanus.
Volcanoes usually have a special shape: They are mountains where the top is missing. Instead, you see a depression there that is roughly round. This depression is called a crater. The edge at the top is the crater rim.
There are about 1500 active volcanoes on Earth, most of them where different continents meet. "Active" means that the mountain is still rumbling and that it can erupt. There are also several volcanoes in Europe, for example on Iceland or in Italy. In Germany there is a mountain range, the Eifel, which was formed by volcanoes. The highest mountain in Africa is also a volcano, Kilimanjaro.