From Things of Interest
In everyday American English Great Britain and United Kingdom are used interchangeably. England too can be used to signify the whole country. There are however key differences between the three.
England, Scotland, and Wales are three constituent countries united under one state, the United Kingdom, on the island of Great Britain. The United Kingdom is the three countries of Great Britain plus the country of Northern Ireland.
The British Isles are Great Britain, the island of Ireland, and a bunch of other small ones like the Isle of Man. The Isle of Man's Lord is the monarch of the United Kingdom but the island is not part of the United Kingdom nor is it a part of the European Union. The same holds true for the Channel Islands of Guernsey and Jersey (but these islands are not part of the British Isles).
Finally there is the Commonwealth of Nations which is a voluntary association of former British (ugh, I do not know the correct word to use) colonies plus Mozambique (which wanted to be politically closer to its neighbors). Each member has the UK Monarch (sans Mozambique) as their own king or queen but remains politically independent.
And you thought the Dutch were confusing.