The atmosphere the air of Earth (and some other planets) and is made of 78% nitrogen, 21% oxygen and 1% of a mix of others. It is the standard base of measurement of atmopsheric density/pressure on varying altitudes and other planets.
The crust is a solid rock that floats above the liquid mantle. It is made up of large tectonic plates that seprate or impact one another, creating trenches and mountains.
The mantle is a large layer of liquid rock 2,900km in size. Convection currents move the molten rock and material around (And that's where volcanoes obtain their characteristic red-hot, molten flowing rock).
Then there's the outher core. It is a 2,300 kilometer liquid ocean of iron and cobalt. Convection currents exist here too as in the mantle, though more frequent.
Ultimately there's the inner core. It's a dense, hot solid ball of iron and nickel. At least 5,400+ Celsius, it is hotter than the surface of the Sun and well, our gravity. Oh, and it is also super dense and has a high pressure.