* Inner Core and Outer Core: These are both made up primarily of iron and nickel, and are located at the Earth's center.

* Mantle: The mantle is a thick layer of mostly solid rock, but with a semi-molten layer called the asthenosphere. It sits between the crust and the core.

Crust: The crust is the outermost layer of the Earth, and it's the thinnest layer. It's composed primarily of lighter elements like silicon, oxygen, aluminum, and iron.

Key difference: The core and mantle are both primarily composed of denser materials, while the crust is made of lighter elements. The crust is also much thinner than the other two layers.