There used to be a British/American difference so that a British billion was a million million (10^12). That has changed in the last 50 years.
Today, all English-speaking countries (and probably most others) use this system:
1 million = 1,000,000
1 billion = 1,000 million
1 trillion = 1,000 billion
1 quadrillion = 1,000 trillion
1 quintillion = 1,000 quadrillion.
There are also colloquially, squillions and zillions, which are a very large (unquantified) amounts indeed.
- For Teachers