King John (1199-1216) was the younger brother of Richard I.
John is usually remembered as being a bad king. He lost many of the English lands in France. He quarelled so badly with the Pope that all church services in England had to be suspended. And angered the English nobles by imposing heavy taxes.
In 1215, the nobles had had enough and rose in open rebellion. King John was forced to sign the 'Magna Carta' - a document which protected people's lives, property and the right to justice, and limited the power of the King to impose taxes.
When sneaky John tried to go back on his promises, the nobles invited the son of the King of France to come and claim the throne. John went on the run. While on the run he famously lost the Crown Jewels in the Wash and died soon after.
The legends of Robin Hood feature King John as the villain.