An incredibly complex, deep, simulation where you terraform new worlds and can control all aspects of the solar system.
How big and hot the sun or suns is, where the planets are, how many moons they have, the planets compositions, etc. You can fast forward and rewind thousands of years at a time in order to see what, if any, life evolves on your planet. As life does start to develop you can go all the way down to the level of the insects or all the way back out of the atmosphere. You can see how the animals live, how weather affects them, how weather systems develop naturally, watch herds of large mammals roam across the land.
The ultimate goal is to set everything up correctly so that over time your planet develops intelligent life that then becomes sentient and intelligent enough to create a civilization.
You can watch and control the primitive intelligence from what we would typically consider cavemen all the way up to planetary explorers.At any point you can take control of the creatures and eventually the intelligent life. At that point all new gameplay systems open up where you can guide the civilization forward and out into the stars.