Volcanos are most commonly found at subduction zone boundaries. This is where the thinner oceanic plates get pulled under the thicker continental plates. As the plates go deeper, they melt and form pockets of magma which can cause mountains to be built and as the hot magma rises, it erupts out of volcanos. One area that this is very evident is along the Andes Mountains in South America. Also, the Sierra Nevada was formed this way and the granite of the Sierra Nevada was formed by the very slow cooling of silica rich magma. Volcanos are also found along spreading centers (under water and at the surface in Iceland) where the thinner oceanic crust is being pulled apart the the hot magma is bubbling up (remember heat rises). A third place is where these are weakness in the earth's crust, commonly called hot spots. Hawaii and Yellowstone are excellent examples of this. These weaknesses are not associated with any plate boundary, but are holes in the earth's crust that move as the plate they are associated with moves. That is why there is a chain of islands in Hawaii. Each island was formed as the hot spot hole allowed magma to come up and form new land above the crust. Yellowstone's movement is noted by where the active geysers are.

Volcanism occurs regularly at two of the three types of plate Boundaries. The most volcanism occurs at diverging plate boundary where plates separate. Volcanoes also form at converging plate boundaries where one plate dives beneath the other at subduction zones.

Usually volcanoes are found near subduction zones, where oceanic crust sinks beneath continental crust. In fact, they're quite often about 200 km inland (on the continental plate) from where the subducting plate sinks below the overriding plate. As the subducting plate sinks, it melts. That rock melt, mixed with water that is pulled down into the subduction zone with the subducting plate, erupts as a volcano at the surface.You have a nice animation to the volcano in the next link:

Volcanoes occur along both divergent sea-floor-spreading plate boundaries and convergent subduction boundaries, but the volcanoes are very different in character between them (the divergent plate boundaries having fissure flow eruptions and the convergent plate boundaries making strata volcanoes). Transform plate boundaries and continent-continent collision plate boundaries do not normally form volcanoes.