Plate tectonics is a theory that was developed in the 1960’s to describe how the earth’s crust is broken up into several pieces or plates which float on earth’s mantle. As the plates move on top of the mantle they often interact with each other. It is estimated that plates move between 1-10 centimeters per year. This movement causes plates to move apart, move under or over each other, push together or slide against each other. The movement of the plates along boundaries can be described as either divergent (spreading apart), convergent (colliding and moving over/under each other) or transform (sliding past or against each other). This interaction can lead to earthquakes and volcanoes!
Source: usgs.gov and public domain wikipedia photo