Plate tectonics is a relatively new science that describes the surface "crust" of the earth as being broken up into many large and small "plates", which are free to move around independently, as they "float" on the molten mantle. They float because the siliceous material that makes up most of the crust is lighter than the ferrous core and mantle.
When plates collide mountain ranges are often thrust up, like the Himalayas or Rocky Mountains. One of the plates may also be "subducted", or sink down underneath one it is colliding with, as seen at the Marianas Trench. Where plates move apart new material from the mantle rises up to fill the gap, as is occurring at the mid-Atlantic ridge. The surface also is returned back to the mantle in subduction zones. Thus the planet's crust is continually "recycled" to and from the mantle. As a result oceanic crust tends to be relatively young while the continental crust is older. Where plates move laterally relative to one another, major earthquake faults develop. The San Andreas fault is one of the most famous of these.
If a plate slowly moves over a "hot spot" below it in the mantle, a series of volcanoes may result as the hot spot melts through successive sections of the plate. This is how the Hawaiian archipelago was formed - and the "spot" is still "hot", resulting in active volcanoes on the newest island.