During a Q&A between a Canadian classroom and Canadian astronaut Chris Hadfield on the International Space Station (seriously, just think about how awesome that is), one kid asked a question that I used to wonder a lot. At the 13:30 mark, he asks:
"Why does the space station have to travel so fast"?
As a kid could understand moving at a high speed to leave the Earth, because I could never launch my Hot Wheels into orbit no matter how hard I threw them. But why would you need the speed when you're already in space? Don't you just float and hang out?
Chris gives a great answer. The space station stays in orbit only because it's going fast - 500 km / minute to be precise!
Chris then gives the example of jumping off of a roof. Obviously gravity brings you back down. The faster you run on the roof, the further you'll travel forward but gravity will still bring you down.
Now, imagine that you were able to run as fast as the space station - 500 km / min. You'd fly off the roof, and as gravity begins to bring you down the Earth would be curving below you. This is essentially what being in "orbit" means - your'e going fast enough that you cancel out the "pull down" effect of gravity .
All of Earth's satellites are in free fall An added bonus is that if you are inside of an object in free fall, you are weightless - hence why Chris is floating about in the video below.
If you can't see the video below, click here. Remember to skip to the 13:30 mark to see Chris answer this question.