Earth is made of big pieces of rock that are constantly moving. Sometimes those pieces of rock rub against or collide with one another. This produces pulses of energy or waves that cause destruction.
If you want to know more information about earthquakes as well as fun games, please go to: http://earthquake.usgs.gov/learn/kids/
If you want to become an earthquake scientist or a geophysicist, please go to: http://earthquake.usgs.gov/learn/kids/become.php