Great Britain is an island located off the northwestern coast of mainland Europe. It is the largest island of the British Isles and includes England, Scotland, and Wales.
The United Kingdom is a political entity that includes Great Britain and Northern Ireland. It was created in 1801 when the Kingdom of Great Britain and the Kingdom of Ireland were merged.
So, the difference between the United Kingdom and Great Britain is that the United Kingdom includes Northern Ireland, while Great Britain does not.