Land volcanoes tend to occur where plates are crunching into each other.  Ocean volcanoes tend to occur where plates are separating.  Both are a part of a system in which crust is created and destroyed. The sea-floor spreading hypothesis says that new crust is created where plates separate in the ocean floor, while old oceanic crust is "recycled" when it collides with continental plates, and is pushed back down into the mantle.  This is how new crust can be created, but the earth's size remains constant.  It also explains why oceanic rocks are younger than continental rocks, and how the Atlantic Ocean is increasing while the Pacific Ocean is shrinking.

Moving plates interact in a number of different ways at their edges.  When plates are colliding, it is called a convergent boundary, when plates are separating it is called a divergent boundary, and when plates are sliding past each other, it is called a transform boundary.  At converging boundaries, the edges may be either oceanic crust or continental crust.  The kind of plates that are part of a collision influences the geological events and features that may be found in the boundary zone.