The Union Flag, also known as the Union Jack, is the flag of the United Kingdom. It retains an official or semi-official status in some Commonwealth Realms; for example, it is known as the Royal Union Flag in Canada. It is also used as an official flag in some of the smaller British overseas territories. The current design dates from the Union of Great Britain and Ireland in 1801.
In 1603, James VI of Scotland inherited the English and Irish thrones (as James I), thereby uniting the crowns of England, Scotland and Ireland in a personal union (which remained separate states). On 12 April 1606, a new flag to represent this regal union between England and Scotland was specified in a royal decree, according to which the flag of England (a red cross on a white background, known as St. George's Cross), and the flag of Scotland (a white saltire on a blue background, known as the Saltire or St. Andrew's Cross), would be joined together, forming the flag of Great Britain and first union flag.
(Source: Wikipedia http://en.wikipedia.org/wiki/Union_jack)