In British English, a billion used to be equivalent to amillion million (i.e. 1,000,000,000,000), while in American English it has always equated to a thousandmillion (i.e. 1,000,000,000). British English has nowadopted the American figure, though, so that a billion equals a thousand million in both varieties of English.
The same sort of change has taken place with the meaning of trillion. In British English, a trillion used to mean a million million million (i.e. 1,000,000,000,000,000,000). Nowadays, it's generally held to be equivalent to a million million (1,000,000,000,000), as it is in American English.
The same evolution can be seen with quadrillionand quintillion. In British English, a quadrillion used to mean a thousand raised to the power of eight (1024), and is now understood to be a thousand raised to the power of five (1015). A quintillion, in British English, used to mean a million raised to the power of five (1030), and is now most commonly held to be a thousand raised to the power of six (1018).
Even higher are sextillion, septillion, octillion, nonillion, and decillion, some of which are not common enough to be included in OxfordDictionaries.com yet.