They are not actually synonymous terms -- England is a nation that comprises part of the island of Great Britain, and is a nation that's part of the United Kingdom of Great Britain and Northern Ireland. However, people in common speaking tend to use "English" as a demonym and "England" as a generic term for the British people and polity (which will tend to annoy the Scots, Irish and Welsh to a great degree if you call them English). It's similar in some respects to the Cold War usage of "Russia" to stand in for the Soviet Union; while Russia was a component state of the USSR it was in many ways first among equals. Similarly, England has been the largest population state of the component British nations. It absorbed Wales in the 13th century; unified with Scotland to form the Kingdom of Great Britain in 1707 (after a personal union of the crowns); and added Ireland to form the United Kingdom in 1801 (later losing much of Ireland in 1922, after the Irish Rebellion and the granting of Home Rule).