The Union Jack, the official flag of Britain since 1801, is actually three separate flags in one, for it combines the English cross of St. George, the Scottish cross of St. Andrew and Irish cross of St. Patrick. England, Scotland and Ireland were originally separate countries, and the Union Jack symbolizes the fact that they now form the United Kingdom, which is another name for Britain.
Although generally known as the Union Jack, this is incorrect because it is the term for a flag flown only from the jack-staff on the bow of the ship to indicate it is a warship. Besides, the correct name of this flag is Union Flag. Through popular use, however, the Union Jack has become the accepted name for the flag. There are no official specifications for the length and width of this flag, but usually an aspect ratio of 1:2 is followed. The colors can be any shades of red and blue.
Union Flag (Wikipedia)