The phenomenon known as subduction occurs when one tectonic plate slides under another one. At the time that this happens the bottom plate is either forced into the Earth's mantle by the top plate or sinks due to gravity. Places where this process commonly occurs are known as subduction zones.
Subduction occurs with both Oceanic crust and Continental crust. The resukts of this process usually makes subduction zones into places with a high riate of volcanic activity as well as a high rate of earthquakes. In addition to this these zones also often cause deformations that can lead to the building of mountains.