Four Countries, One State
The United Kingdom is a single sovereign state, but it is made up of four countries: England, Scotland, Wales, and Northern Ireland. This is one of the things that makes the UK unusual. It is one nation on the world stage, with one government in London, yet inside it are four distinct countries, each with its own history, culture, and identity.
How They Came Together
The four countries joined the union at different points in history rather than all at once. England and Wales were legally joined first. Scotland came together with England through a union of their parliaments. Ireland was added later, and then most of Ireland eventually became a separate independent country, leaving only Northern Ireland inside the UK. The full official name of the country reflects this story: the United Kingdom of Great Britain and Northern Ireland.
Great Britain Is Not the Same Thing
It is easy to mix up the terms. "Great Britain" refers to just three of the four countries: England, Scotland, and Wales, which all sit together on one large island. The "United Kingdom" includes those three plus Northern Ireland. So Great Britain and the United Kingdom are not the same thing, even though people often use the names as if they were.
Source
This article was written using information from Wikipedia.