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The Crust
The crust is the very outer layer of Earth. When you walk on dirt or in a field, what you're walking on is the Earth's crust. The crust is mainly made up of alumino-silicates. The continental crust, that which makes up dry land, is anywhere between 35 and 70 km thick, whereas the oceanic crust, which makes up the sea floor, is between just 5 and 10 km thick.
The Mantle
The mantle is divided into two sections, the upper and the lower mantle. The lower mantle is identified by having a higher density than the upper. Both sections together are 2,900 km thick and make up 80 percent of the Earth's volume. It is predominantly composed of ferro-magnesium silicates. The mantle isn't molten, but it nears melting point, once you get 100 to 200 km below the planet's surface. The mantle contains convective material, which circulates heat and may be what causes movement in the tectonic plates.
The Outer Core
The outer core is 2,300 km thick. It's made up of molten, liquid metals -- iron and nickel -- and minor sulphur. Geophysicists believe that the outer core is responsible for controlling the Earth's magnetic field. Although scientists can't actually analyze material from the inner core first hand, they can make the assumption that it's liquid based on the behavior of shear and compressional waves when they're passed through it.
The Inner Core
Like the outer core, this compositional layer of the Earth is made up of metal. However, unlike the outer core, it's solid metal. The inner core is composed almost completely of iron, but around 10 percent of it is thought to be sulphur, nickel or oxygen. It's 1,200 km thick, which makes the two parts of the core combined over half of the diameter of the Earth.