PLATE TECTONICS
Plate Tectonics is a theory to explain widespread evidence for continental drift and sea floor spreading.  The earth's crust is broken into several plates each moving in various directions.  Plate movement is the result of convection currents in the mantle.  Adjacent plates can be moving in three ways; 1) away from each other (Divergent Plate Boundary) 2) Toward each other (Convergent Plate Boundary) or 3) Sliding past each other (Lateral or Transform Plate Boundary).

1. Divergent Plate Boundary - This occurs when two plates are spreading apart, and new crust is being formed in the middle.  Also called a mid-ocean ridge boundary because divergent boundary's usually occur in the middle oceans.  At the middle of these boundary's new ocean floor is being created and pushes apart the 
plates. This picture shows a spreading center and a subduction zone.

Links to Sites with Good Examples - Link 1    2    3

CONVERGENT PLATE BOUNDARIES

Convergent Plate Boundary - Subduction Zone (ocean - continent)

B. Ocean - Continent Boundary - When two different plates collide the oceanic crust is always subducted beneath the continental crust because oceanic crust is more dense.  This is called a SUBDUCTION ZONE when one plate sinks below another (Example is the Nazca plate diving below the South American Plate which creates the Andes mountains)

Convergent Plate Boundary - Uplift Zone (continent - continent)

A. Continent - Continent Boundary - When two continental plates come together (Example: the Indian Plate is crashing into the Asian Plate which is forming the Himalayan mountains where mount everest is located.

Convergent Plate Boundary - Island Arc Subduction (two ocean plates)

C. Ocean - Ocean Boundary - When two oceanic plates collide one plate sinks below the other and forms a chain of volcanic islands in the middle of the water (Example: the islands of japan)

THE THIRD POSSIBLE PLATE BOUNDARY
Lateral or Transform Fault Boundary

3. Lateral or Transform Plate Boundary- When two plates are sliding past each other, moving in opposite directions, The example for this type of boundary is the San Andreas Fault line. 

PLATE TECTONICS ARE DRIVEN BY CONVECTION CURRENTS IN THE MANTLE

PLATE TECTONICS ARE DRIVEN BY CONVECTION CURRENTS IN THE MANTLE OF THE EARTH.  AS HOT MAGMA RISES FROM THE LOWER MANTLE IT SEPARATES NEAR THE CRUST CAUSING MID OCEAN RIDGES, WHEN MAGMA SINKS IT BRINGS THE CRUST DOWN WITH IT CAUSING SUBDUCTION ZONES. NOTICE HOW THE CONVECTION CELLS RELATES TO THE EARTHS CRUST.

Hot Spots - occur at points where magma comes up through the crust in the middle of a plate, see this animation!

http://www.scienceprof.com/swf/Hawaii.swf