Because the molten magma in the mantle is constantly moving, so are the plates. The places where plates meet one another are sometimes called faults. Basically, there are three types of movements that describe what happens along these faults:

Convergent plate movement occurs when two plates are headed right for each other. When the plate types are the same, they tend to form fold mountains (continental) or trenches (oceanic). When they are different kinds, the oceanic plate tends to sink below the continental, then it melts into magma in the hot mantle. This is called subduction.

Divergent plate movement occurs when two plates are flowing away from one another. This generally occurs when a crack in the crust allows magma from the mantle to escape to the surface. This allows the plates to flow away from one another, while the magma continually comes up, cools, and forms new crust, and sometimes landmasses.

Transform plate movement occurs when two plates are "rubbing" up against one another and floating in different directions because of the currents in the magma of the mantle. Neither plate drops below the other, because they are sliding along one another.