Earthquakes is the shaking of the Earth caused when an abrupt shift of rock along a fracture in the Earth. The tectonic plates are always slowly moving, but they get stuck at their edges due to friction. When the stress on the edge overcomes the friction, there is an earthquake. In a fault, rocks moving in different direction squeeze and have no place to go. Pressure keeps building up until it reaches its maximum amount of pressure the rock slips until all of the pressure is gone. The release of energy causes a seismic wave that shakes the ground. Basically underground rocks are rubbing on one another; it breaks when all the pressure is built up.
Elastic Rebound- Theory of earthquakes that envisages gradual deformation of the fault zone without fault slippage until friction is overcome
Focus- Spot underground where the rock breaks\
Epicenter- The place right above the focus (Ground Level)
Seismic Wave- Elastic wave generated by an impulse such as an earthquake 
Fault- Cracks in the Earth where section of a plate are moving in different directions