How does the Space Station stay put in orbit?

The International Space Station (ISS) is not stationary, but is in orbit around the Earth, making one orbit about every hour and a half. It's forward speed almost perfectly balances the gravitational pull of the Earth downward with the centripetal force outward. Occasionally the ISS must be given a small boost in altitude because at it's normal altitude there is still a very thin, tenuous part of our atmosphere, which acts to slightly brake the forward speed of the ISS, causing it to very slowly descend. If this descent was to go unchecked, then the ISS would eventually re-enter the atmosphere and break up/burn up.