Whip out your maps, folks. Let’s start with pure geography: Great Britain (also known as Britain) is an island in between the North Sea and the English Channel. Ireland is an island to the west of Britain. Both Great Britain and Ireland are a part of the British Isles, a group of over 6,000 islands off the northwestern coast of Europe.
Things get complicated when we start talking politics. The island of Great Britain is made up of the countries of England, Wales, and Scotland. The island of Ireland is made up of Northern Ireland and the Republic of Ireland. Two islands: three countries on one, two on the other. Got it? Let's move on.
The United Kingdom of Great Britain and Northern Ireland, or the United Kingdom for short, is a sovereign state made up of the countries of England, Wales, Scotland, and Northern Ireland (so: all countries on the island of Great Britain, plus Northern Ireland from the island of Ireland). All of the countries in the United Kingdom are run by Parliament and are bound to the crown, though the Parliament delegates some decision-making to the Scottish Parliament, the Assemblies of Wales, Northern Ireland, and London, or to local authorities for “devolved matters” such as education and housing.
The Republic of Ireland, however, operates as a separate sovereign state and has its own relationships with the European Union, the United Nations, and other international organizations. It gained independence from what was once the United Kingdom of Great Britain and Ireland (now the United Kingdom of Great Britain and Northern Ireland) in 1922.
Here’s a handy map from Encyclopedia Britannica to help illustrate all of this:
So, in sort: Great Britain (or Britain) is an island; England is a country on the island of Great Britain; and the United Kingdom is a sovereign state made up of the countries of Great Britain—England, Wales, and Scotland—and Northern Ireland.