Planet Structure
To understand the remaining geologic processes, it will help to understand how planets are created in the first place.
All the planets and moons in our solar system formed at the same time, by a similar processes. Since we are only worried about terrestrial planets and moons (not gas giants like Jupiter), we will only worry at all about the formation of rocky worlds.
Once upon a time, about 5 billion years ago, our sun was just forming and there were no planets. There was just a big cloud that was collapsing togther. The cloud was mostly made of hydrogen but it had a bunch of other stuff in it as well (like helium, carbon, oxygen, etc.). Gradually, it formed a spinning disk with most of the material at the center and a bunch of stuff orbiting. Over time, the material in the spinning disk started to clump together, forming bigger and bigger pieces. When piecees are big enough we call them protoplanets. They have enough gravity to attract nearby material. This nearby material rains down on the protoplanet. As the material crashes into a protoplanet, it heats the protoplanet. Protoplanets in similar orbits might also violently collide, producing a lot of heat. Or, protoplanets in a similar orbits going around the sun might start to orbit one another as well. This forms a double planet or a planet with a moon. A moon can also form when two large protoplanets collide and the collision sends a lot of material into orbit around the new, combined protoplanet. The material in orbit starts clumping together and eventually can form a moon.
All of this crashing causes so much heat the protoplanets and planets completely melt. The entire body, including all the solid rock, liquifies. These hugh balls of molten rock are held together by gravity just as a gas giant like Jupiter is held together. In this molten planet, the heaviest stuff falls to the center. That is how terrestrial planets develop an iron core and a surface made of much lighter rock. Geologists call this process differentation.
Eventlually, the solar system is left with a couple of terrestrial planets and their moons in stable orbits.
As this was happening, the material at the center of the very center of the solar system kept getting denser and denser (due to gravity) until nuclear fusion was ignited in its core and it became a star and our sun. So our solar system begins with a young star and several planets (and their moons) in orbit. The terrestrial bodies are liquid, molten, balls of rock. Since the planets are liquid, it was easy for different kinds of rock to separate. The heaviest, densest material (like iron) fell to the center of each planet, while the lightest stuff floated on the surface like pond scum.
The heat to melt the planets came from the collisions. After most of the collisions had occured, the planets lost this source of heat. Over time, they naturally cooled. And, just like a pie that you take out of the oven, the outside cools first while the inside stays warmer. When the surface cooled enough, the liquid surface froze into solid rock. The very same thing happens when lava pours out of a volcano as a liquid and it quickly cools into new rock. So, on these young planets, a thin crust forms over the liquid beneath. We call the solid surface the crust and the molten layer below it the mantle. The heavist material that went to the center of the planet is called the core. As time marches on, the inside of planets contiue to cool down which means more liquid magma will turn into solid rock and the crust gets thicker and thicker.
That is the story of how terrestrial planets and moons form. This is how our Earth formed. You might think a solid crust basically floating on a liquid sounds crazy and not very stable. Well, you'd be half right. It isn't very stable. And it is made worse because the mantle shifts around. Hotter parts rise, cool and then sink again. This is why Earth and the other terrestrial bodies suffer from earthquakes and volcanoes until the crust becomes very thick.