Great Britain is one shortened version of the name of a country with the official name United Kingdom of Great Britain and Northern Ireland. The term "Great Britain" technically refers to the island containing England, Scotland and Wales, so calling the country the United Kingdom is more accurate.
Although united under one system of government, a constitutional monarchy, Northern Ireland, England, Wales and Scotland each have their own cultures and established religions. Wales, Ireland and Scotland also have their own traditional languages, though English is the universal language. The United Kingdom once had territories worldwide, but the country's dominion has shrunk, and most territories that do remain have their own governments in place.