There are four main layers in the earth.
Crust: This is the part we see, and includes both the continents and the ocean floor. The thickness of the crust varies from 35 to 70 kilometers in the land and 5 to 10 km in the ocean basins.
Mantle: Below the crust is the mantle, which is divided into upper and lower sections. This layer is nearly 3000 km thick.
Lithosphere: This term is used to describe the crust and rigid top part of the upper mantle. The lithosphere is tough and brittle (it can break in earthquakes). This is the part of the earth that forms the tectonic plates.
Asthenosphere: This is the lower part of the upper mantle below the lithosphere. It is very soft and plastic (squishable); although it is solid, it flows slowly.
Outer Core: This is below the mantle and consists of liquid iron, nickel and sulphur. It ranges from about 3700 - 4300 degrees C, and is about 2200 km thick. This flowing metal creates the earth's magnetic field.
Inner Core: This is the center of the earth, and is made mostly of iron. It is under such extreme pressure that it is solid, even though it gets as hot as 7200 degrees C. It has a radius of about 750 km.