Generally accepted accounting principles (GAAP) are a set of rules, accounting principles and standards that are used in a particular region or country.
GAAP are drawn from several sources. Standards (such as IFRSs) are the most obvious source, however there are others. Company law (specifically, the Companies Act) is also very important in the UK.
The Accounting Standards Board sets UK accounting standards, which are now based on IFRSs. Company law changes less frequently.
US GAAP is set by the Financial Accounting Standards Board (FASB).