Volcanism is the arrival at the surface of a planet or natural satellite of a liquid called lava or magma from the molten interior of the body. The outpouring of magma is often accompanied by gaseous emissions.
The morphology created by volcanism can vary a lot; it depends on the viscosity of the magma and its gas content. A very fluid lava will flow like water without creating relief and leaving vast expanses of lava, or volcanoes with shallow slopes; conversely, a very viscous lava will accumulate on pouring out, making a dome or spine. A high gas content will cause the magma to pulverise, and the fragments of magma will accumulate at the exit points, producing cones, or else will be scattered over the whole region. All these possibilities exist, but in nature there are generally intermediate ones with association of several types, producing the classic volcano.
In the solar system, planets of the Earth type and most of the large satellites have volcanism. The volcanism on Mercury and the Moon is formed mainly of flows and lava sheets coming out of fissures that have since filled up, almost without a "volcano" in the traditional sense: this is volcanism that has been extinct for at least 3 billion years.
Mars mainly has volcanism consisting of flows and sheets, but there are also a few giant volcanoes made of slightly less fluid lava accumulating around the exit point of the magma: the most spectacular is Olympus Mons, the largest volcano in the solar system with a diameter of 600 km and a height of 26 km. Earth and Venus have all types of volcano, and they are still active today. But the champion for volcanic activity in the solar system must be Io, one of the four main moons of Jupiter, which offers a wide variety of volcanic structures.