Exactly as the title says.

You can check your Version of the game by looking at the top left of the computer screen on the beginning of the game. It should say v0.8.12 if you are on the original and v0.9.4 if you have the beta build on.

What are the main differences?

Original is the original game build. Beta has extra levels and a hub that interrupts the flow of the original so different timing all together