A billionaire is somebody who has at least one billion units of currency (for example, British pounds £ or American dollars $). It is not clear how much a billion is. In Europe a billion is one million million (1,000,000,000,000) with 12 zeros. However, in the United States a billion is one thousand million (1,000,000,000) with 9 zeros. Some people in the United Kingdom, especially banks, also use the American way.
"Milliard" is used for 1,000,000,000 in countries that use one million million for billion.