The Union Jack resulted from combining different flags. Here is a short overview of the three different flags:
The diagonal white cross on the blue background represents St. Andreas, the patron of Scotland. It is the national sign of Scotland since the 12th century.
The Patrick cross is the national sign of Ireland and consists of a red diagonal cross on a white background.
The holy Georg acts as a patron of the knighthood since the beginning of the 15th century and is the national sign of England. It is represented by a red cross on a white background.
Through the gradual consolidation of England and Scotland in 1603, a flag was created that incorporating both original flags.
Since 1801, when Ireland joined the alliance, the third flag was merged into the existing one to create the flag we know today as Union Jack.
And here is the Union Jack again, the way it is found all across Great Britain today.