From Pompeii to Mount St. Helens, volcanos have held a special place of fascination and awe throughout civilization.
Volcanos are in essence mountains that exist over a pool of magma, or molten rock. When the pressure builds and perhaps plates shift within the earth, that molten rock gets forced to go up and out the mountaintop. Magma that is on the surface is then called lava, and the incredibly hot rock in liquid form then cascades down the mountain in bright red streams.
Not only the lava - which reaches over 2,000F - comes out of the volcano. There are also poisonous gasses, steam, and clouds of dust that come along with an eruption. In many cases it is these gasses and dust that cause the most damage.
When a volcano cone exists but no longer has magma beneath it, it is called extinct. If the magma exists but is not under any pressure to erupt, the volcano is called dormant. If the volcano is actively emitting gasses and perhaps lava, that volcano is called an active volcano. There are many active volcanos in the world. In fact, in Costa Rica you can book a hotel room at the base of the Arenal Volcano and watch the lava bubble.