With the Olympic Games being held in England there is some confusion as to the difference between England, Great Britain and the United Kingdom
England (includes Wales)
Great Britain = England, Scotland and Wales
United Kingdom = England, Scotland, Wales and Northern Ireland (Southern Ireland is the Republic of Ireland)
British Isles = England, Scotland, Wales, Ireland and the surrounding Islands including the Isle of Man and Channel Islands
See more info here
The Union Jack is made up of the English flag (red cross) the Scottish flag (white diagonal on a blue background) and the Irish flag (diagonal red cross on a white background).
Wales had previously been combined with England and the Welsh flag is not depicted on the Union Jack.