County Durham is often called the Land of the Prince Bishops – it even says it on the signs as you enter. There is a long and bloody history behind it but it basically boils down to this:
William the Conqueror realized that England was a bit too big to control on his own – especially keeping those pesky Scots out. He therefore gave over this control to his bishop based in Durham and Bishop Auckland. He gave them nearly all the powers that he had hence them becoming known as the Prince Bishops.
The longer answer is entwined in the history of the Anglo-Saxon kingdom of Northumbria which is now Northumberland and County Durham.