The Union Flag, also more commonly known as the Union Jack, is the national flag of the United Kingdom.
This British flag is made up of a combination of the three heraldic crosses of England, Scotland and Ireland (however since 1921, only Northern Ireland has been included). These three Kingdoms are united under one Sovereign. Wales is classed as a Principality, not a Kingdom, and was already united with England, therefore it is not included. The three heraldic crosses are the red cross of St. George, the blue and white saltire of St. Andrew and the red and white saltire of St. Patrick.
ClickSouvenirs.com have developed this infographic to show the history behind how the Union Flag was designed, combining the three older national flags.
Source of infographic : http://www.clicksouvenirs.com/infographics/history-of-union-jack-flag.asp