In English, a “billion” is 1 000 000 000 (a thousand million).
This has always been the case in US English.
In British English, in the past the word “billion” meant a million million. If we wanted to refer to a thousand million, we simply said “thousand million” or more rarely “milliard”. But in 1974 we officially adopted the US practice of using “billion” to mean a thousand million.
The word “milliard” has since gone out of use in British English. It never existed in US English.
Much of the confusion over the usage of these words derives from variants of the word “milliard” remaining in common usage (and meaning a thousand million) in other European languages, e.g. Spanish millardo, French milliard, German milliarde, Polish miliard and Russian миллиард.
Remember these words translate into “billion”. And there is no longer any distinction between British and US usage.
For more information see http://en.wikipedia.org/wiki/Long_and_short_scales