A billion, in America, is a thousand million. That would be written

1,000,000,000
If we wanted to make a book with a billion dollar signs, printed 1000 per page as before and with pages printed on both sides, our book would be 500,000 pages long.Now that's a very long book!

Interestingly, in England, the British define a billion as a million million. That would be 1,000,000 times 1,000,000 which would be written 1,000,000,000,000. The British book would be 5,000,000 pages long.

1,000,000,000,000
You can find out more about the British numbering system at the following website called Numbering Systems and Place Value:

http://www.mazes.com/numberingsystems.html