Volcanoes are found along destructive (subducting) plate boundaries, constructive (divergent) plate boundaries and at hot spots in the earth's surface.
Volcanoes occur when magma (melted rock) from within the Earth's crust reaches the Earth's surface, and are generally located along the boundaries of tectonic plates (large segments of the Earth's crust). Volcanic activity is a function of the plate boundary type; therefore, different types of volcanoes form at different plate boundaries.
Those that form volcanic mountain chains along convergent plate boundaries, where one plate is being pushed (subducted) below another, such as the Cascade Range in the Pacific Northwest,
Those that form at "hot spots" in the middle of plates, such as the Hawaiian Islands, and
Those that form where two plates are being pulled apart and, as a result, the crust is thinning and new crust is being formed, such as in Iceland the along the Mid-Atlantic Ridge.