1,000 * 1,000,000 = 1,000,000,000 = 10^9 = 1 giga-
Thus, a US billion has 9 zeroes.
This is called a Milliarde in German, where Germans mean a EuropeanBillion by the word Billion.
The UK spelling of the old name is milliard (contrast with the German spelling). The series then continues with billion (tera, US trillion), billiard (peta, US quadrillion, source of the name of a pool-oid table ball-game), trillion and presumably trilliard (not Trillian ;^)
The BBC World Service uses the term thousand million so that everyone will understand what is being referred to.
However, these names, though still current in Europe, are archaic in England (and possibly also in the more civilized countries of Britain). Rupert Murdoch [an ex-Aussie - Australia was AFAIK the second country to take up the AmericanBillion] uses the US terms, so his vassals do, too.
As I understand it, the US usage changed when journalists had exhausted the wow-value of blabbing on about millionaires and a few of that nation's most wealthy came to have control over a milliard of dollars; the journalists, it seems, either did not understand what a billion was, or found the word milliardaire inadequately sensational. AnyOldWhichWay?, they ended up referring to these giga-dollar plutocrats as billionaires; and, as they say, TheRestIsHistory?.
See also BritishBillion