Earthquakes are caused when rocks which are underground break along a fault line. This causes a release in energy which in turn causes a seismic wave(s) which make the ground shake.

When two blocks of rock happen to rub against each other, much like regelation in ice, they tend to stick to each other. Now while they're stuck to each other, they aren't moving but they are under a lot of pressure. When the pressure on these rocks becomes too much the rocks will break and continue slide along each other until they stick to each other again. The break and the rocks sliding on each other untill they stick again is what causes the earthquakes.

This is not the only way earthquakes can occur though, they can also occur when large explosions occur underground like when people are trying to dig new tunnels in mines. Its not nearly as powerful as a the first case, but it could still make the earth shake.

Source: www.geo.mtu.edu/UPSeis/why.html