Original question: Click me

How exactly does a collision engine work?

This is an extremely broad question. What code keeps things bouncing against each other, what code makes the player walk into a wall instead of walk through the wall? How does the code constantly refresh the players position and objects position to keep gravity and collision working as it should?

If you don't know what a collision engine is, basically it's generally used in platformer games to make the player acutally hit walls and the like. There's the 2d type and the 3d type, but they all accomplish the same thing: collision.

So, what keeps a collision engine ticking?