If you're differentiating between Russia, the Soviet Union, and the USSR, then I give up. I was never able to get a straight answer on what the difference was between the UK, England, and Britain, either. 
eh? It's easy.
The United Kingdom (of Great Britain and Northern Ireland, to give the full title) is the country consisting of Scotland, England, Wales and Northern Ireland, first established by the union of the Scottish and English crowns in 1603, when James VI of Scotland becames James I of England (who lacked a natural successor to the throne), followed by the 1707 Act of Union. All share the same national government, although Scotland has a devolved parliament, Wales a slightly-less devolved National Assembly and Northern Ireland a suspended (due to Sinn Fein - DUP bickering over the IRA ceasfire)
England is the largest of those 4 composite 'countries', with a population of about 50m (out of 55-58 or so total).
Britain is a general term referring to the UK; it descends from the roman province of Brittain (NB: kind of simplified etymology here). The British Isles refer to both the UK and Eire.
