There are several main types of volcanic cones: shield, cinder, composite, and spatter. All volcanoes are caused by ruptures in the Earth's crust which allow magma, hot gas, and chunks of debris to emerge, sometimes explosively. The cone of a volcano is a buildup of debris around the vent in the Earth's crust; cones are classified by the substances they contain and the way in which they form. Different volcanic cones also act differently, so ascertaining the cone type of a volcano is extremely important, especially in populated areas.
When most people think of volcanoes, they think of cinder cones, also sometimes called ash cones, which have a classic volcano shape. These cones are made from loose volcanic fragments which pile up steeply around the vent as they are ejected. Cinder cones are very simple, and they can form extremely quickly; Paricutin in Mexico, for example, grew from nothing to a 1,100 foot (336 meter) volcano over the course of a year. Cinders can vary widely in size, and commonly a cinder cone has several different types of volcanic material.
A shield volcano forms when magma moves slowly, creating a large, gently sloping volcanic cone. A well known example of a shield volcano is Mauna Loa in Hawaii. The lava emitted by a shield volcano has a low viscosity, so it pours out readily, and these volcanoes may form slowly over a long period of time, with fresh lava deposits layering on top of older ones. Volcanic cones of this type have a very low profile, looking rather like a giant warrior's shield lying on the ground.
Volcanic cones of the spatter type are formed by thick lava and magma which erupts violently around a vent, creating gobs of material which fuse together as the lava cools. Spatter cones are characterized by very irregular surfaces, caused by large chunks of lava and other material. Fans of the Lord of the Rings series may be familiar with spatter cones, as Mount Doom is a classic example.
Composite volcanic cones are caused by alternating layers of magma, rock fragments, and ash. They often erupt violently and unpredictably; Vesuvius, Mount St. Helens, and Mt. Shasta are all examples of composite cones. Composite cones can also cause lahars, mudflows of volcanic ash which can be quite devastating.
Finally, a bulge of magma under the Earth's surface which has not fledged into a volcano is known as a lava dome. Lava domes often form in or near volcanoes, but they can appear in other areas with high geologic activity as well. Lava domes could be viewed like pimples on the Earth, and like pimples, they can pop, sometimes quite explosively with a shower of incredibly hot gases, magma, ash, and chunks of rock.