The overall structure
The bit we live on is called the crust, it is very thin, although about 20km.
Below that is the mantle. It has all of the properties of a solid.
At the centre of the earth is the core, which we think is made up of iron and nickel. The inner core is solid, but the outer core is a liquid.
The crust and upper part of the mantle are cracked into tectonic plates.
They don't stay in one place, but drift in convection currents.
At plate boundaries, they may slide past each other which sometimes causes earthquakes.
They are difficult to predict.