“God Save the Queen” (or “God Save the King”, depending on the gender of the ruling monarch) was first publicly performed in London in 1745 to support King George II after he was defeated in a battle in the Jacobean uprising that started that year. The song was used to boost morale and the forces loyal to George II would go on to defeat the Jacobites the following year. The song came to be referred to as the national anthem from the beginning of the nineteenth century.
The words and tune are anonymous, and may date back to the seventeenth century. There are various claimants to authorship of both the words and tune, the words can be found as early as 1545, when the watchword at night was “God save the King”, the reply was “Long to reign over us.” The authorship of the melody has been claimed by many, including John Bull (the author of the earliest piece of music that resembles the work), Henry Carey, Henry Purcell, and Joseph Haydn (although he probably borrowed the tune upon hearing it in London.)
There is no authorised version of the National Anthem as the words are a matter of tradition. The anthem has also never been officially declared as the national anthem of the country, the royal anthem (as this technically is) is used as the national anthem as a matter of tradition, but this is also due to the unique constitutional situation in the United Kingdom, as the nation doesn’t have a formal constitution. The words used are those sung in 1745, substituting ‘Queen’ for ‘King’ (and female pronouns with male ones) where appropriate. On official (and most other) occassions, the first verse only is sung, on a small number of occassions, the third verse is heard as well; very rarely is the second verse heard due to its militaristic nature. There exist many other verses, some dating as far back as the first three verses, but the first three are what can best be represented as the “standard” British national anthem.
The British tune has since become one of the world’s most recognizable anthems, and has has been used in other countries - as European visitors to Britain in the eighteenth century noticed the advantage of a country possessing such a recognised musical symbol - including Germany, Russia, Switzerland, the United States (where use of the tune continued after independence as the patriotic song “My Country, ‘Tis of Thee” and one of several unofficial anthems before 1931), and even today by Liechtenstein and as the royal anthem of Norway. The song also was an influence on early anthems used in the Kingdom of Hawaii. (One might say that because of this fact, that the United Kingdom was the creator of the concept of a “national anthem”.) Some 140 composers, including Beethoven, Haydn and Brahms, have used the tune in their compositions.
“God Save the Queen” also serves as the royal anthem for most Commonwealth countries, such as Australia and Canada. (Governors-general of Commonwealth countries usually have bits and pieces of the national anthem strung together played as their anthem.)