Grammar is essentially the way the English language fits together. Unlike some other languages, its rules aren't governed by a single body, but are rather a collection of commonly-accepted rules. Because English has developed in several places simultaneously, these rules are sometimes disputed between countries or regions.
For this wiki, the rules that generally obtain in Britain are preferred. There are any number of sites on the internet that will help you discover the basic rules of grammar. However, the rules of the British Council are seen as definitive, in the case of any disputes.
Since British grammar standards overwhelmingly apply in Australia, New Zealand, India and most parts of the old British Empire, the users with the greatest difficulty grasping the subtleties of British grammar are inevitably North Americans.
Here are a few "quick guides" to the differences between British and American grammar: