The boundary between two plates is a fault zone, and it is in these zones that earthquakes typically occur. There is a huge amount of friction that builds up on these fault lines, as the plates move against each other. The tension has to be released at some point, causing an earthquake (see image to right). To see what this is like, try this little experiment. Press your hands together tightly. Now, try to slide one forward. It is difficult to move, at first. At some point, though, your hand shoots out, as the pressure created by your arm is greater than the friction between your hands. This is exactly what happens on a fault line.
A prime example of a fault would be the San Andreas Fault, in California. This fault marks the boundary between the North American and Pacific plates. There has been tension building up in a section of this fault, near Los Angeles, for around 300 years. Because of this, it is possible that a newly discovered type of earthquake will hit is area, and soon. Supershear earthquakes are the geological equivalent of a sonic boom. They travel much faster than seismic waves were thought to be able to. They could be devastating, if they hit major cities like LA or San Francisco. There is a lot more to earthquakes, and I'll try to elaborate further in another post. For now, though, check out TheTech for more about earthquakes in general.
Credit to New Scientist for information of supershear earthquakes, and to the TV show How the Earth Was Made.