Convergent Plates: When two continental plates converge or collide, the crust is squished together, folding and buckling and forming mountains. This is just one way mountains are formed.
Subduction Plates: When one continental plate and one oceanic plate collide, the more dense oceanic plate is subducted or pushed under the continental plate. As it plunges deeper into the mantle, the rock of the oceanic crust melts, becoming part of the mantle. An ocean trench forms at the plate boundaries. Volcanoes and earthquakes may happen here. In the case of volcanoes, molten rock rises until it erupts at the surface. 
Divergent Plates: When plates spread apart or diverge, magma rises up from the mantle and fills the gap, creating new crust. As this continues, volcanic islands may form at the surface.
Transform Plates: When plates grind against and past each other in opposite directions earthquakes may strike along these boundaries.
This map shows the continents and oceans and the plates on which they move.
