The flag of the United Kingdom, commonly known as the Union Jack (a term that perhaps derives from the use of the Union Flag on the jack-staff of naval vessels), embodies the emblems of three countries under one sovereign. The emblems that appear on the Union Flag are the crosses of three patron saints:

• the red cross of St George, for England, on a white ground;
• the white diagonal cross of St Andrew, for Scotland, on a blue ground;
• the red diagonal cross ascribed to St Patrick, for Ireland, on a white ground.
  

The final version of the union flag, including the cross of St Patrick, appeared in 1801, following the union of Great Britain with Ireland. The cross remains in the flag, although now only Northern Ireland is part of the United Kingdom.

Wales is not represented in the Union Flag because, when the first version of the flag appeared, Wales was already united with England. The national flag of Wales, a red dragon on a field of white and green, dates from the 15th century and is widely used throughout Wales.

The Union Flag should be flown with the broader diagonal band of white uppermost in the hoist (near the pole) and the narrower diagonal band of white uppermost in the fly (furthest from the pole).

There is more information about the British flag on the British Monarchy website.