Gravity wells. I only realised in my 20s that the only reason satellites can orbit the Earth without crashing into the ground is by going sideways really, really fast. So as they inch closer to the ground, they also travel parallel to the ground fast enough so that they stay approximately the same height from the ground.
>>pgt+(OP)
This is Newton's Cannonball. Honestly, I've found the best way to learn more about orbital mechanics is with a simulator - Kerbal Space Program is a fun version.