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Think of what happens when you throw a baseball towards a batter. The baseball will quickly fall towards the earth. The faster you throw it, the farther it goes. Imagine that you can throw it so fast (18,000 mph, or so) that it falls at the same rate as the earth's curvature. It keeps falling, but it never gets any closer to the earth. You have just launched it into orbit.

One problem is that air resistance will slow it down, so it will quick;y come crashing down again. However, if you can get it up high enough, where there is no air, there will be little to slow it down, so it will stay in orbit indefinitely. The faster it goes, the higher its orbit will be.

To get it back down, you simply have to slow it down. The astronauts use "retro rockets", rockets that point in the reverse direction of travel so that they actually make it go slower.