Lithospheric plates span vast areas and are relatively thin. They interact with one another mainly at the plate boundaries. There are three possible interactions, depending on the motion of two interacting plates relative to one another.
If the two plates are moving toward one another, we say they are converging. This creates a situation where the material (rock) that makes up the two plates is getting pushed together. Two different situations occur at convergent boundaries. If the crust on one or both plates is oceanic crust, subduction occurs. Subduction means the plate on one side passes beneath the plate on the other side and bends down into the asthenosphere below the other plate. Continental crust is both less dense and thicker than oceanic crust. It does not subduct—it is apparently too lightweight and thick to slip beneath another plate and down into the asthenosphere. At a boundary having continental crust on both sides, the rock is pushed together and piles up. This creates high mountain ranges such as the Himalayas, which also are subject to violent earthquakes because of the movement of the plates beneath them.
If plates are moving apart, they are diverging. New crust is created in the opening gap when molten rock or magma wells up from beneath, cools, and solidifies. A divergent boundary between oceanic plates is a mid-ocean ridge, and a divergent boundary between continental plates is a rift. Africa's Great Rift Valley is a geologic feature caused by plate divergence. Another famous rift occurs in Iceland, where earthquakes and volcanoes associated with the rift are commonplace.
Finally, plates can be moving in opposite directions past one another. This is called a transform boundary. A famous example is the San Andreas Fault system on the west coast of North America. The western side of the fault is moving northwest, while the eastern side is moving southeast. The plates move evenly but the rock at the boundary does not. Friction keeps the rocks from sliding, so they bend, building up stress as they are bent more and more. From time to time the stress exceeds the strength of friction, and the rocks slip, causing an earthquake such as the famous one that devastated San Francisco in 1906.
Earthquakes occur at all three kinds of plate boundaries, as rocks are moving and will be pinned for a time by friction and then slip. Volcanoes occur at divergent and convergent boundaries (although they are different kinds of volcanoes).