One thousand millions equal one billion. According to Oxford Dictionaries, this equivalence has always been true in American English, but British English used to equate one million millions with one billion.
Sometimes, the British press uses "one thousand million" as an alternative way to indicate one billion. This alternative term is helpful to speakers of other languages, such as Spanish, for whom the term "billion" actually refers to one million million, not one thousand million. "Milliard" is also a British term for one thousand million, although it has been gradually phased out among the British scientific community.
In the United States, one billion is the general standard for representing one thousand million.