First-world countries were originally North Atlantic Treaty Organization (NATO) members and their allies, second-world countries included communist-socialist states loyal to the United Soviet Socialist Republics (USSR) during the Cold War, and third world countries were undeveloped nations that didn't align with either bloc after World War II. The terms were created by French demographer Alfred Sauvy in 1952.
In general, first-world countries were industrialized, democratic nations while second-world countries were just as developed as their first-world counterparts. First-world countries included the United States, most of Western Europe, Japan and Australia. Second-world countries were the USSR, Eastern Europe and China. The third world included developing countries in South America, Central America, Africa and Asia.
Since Sauvy's designations, these alliances have changed and developing nations have become more industrialized. Investopedia explains more modern contexts in that the first world is characterized by stable, democratic governments with economic viability and high standards of living. Second-world nations are less developed than first world nations but more developed than third world countries. Examples of contemporary second-world countries include South America, Turkey, Thailand and South Africa. Third-world countries are marked by instability and extreme poverty.
Some demographers describe a fourth world in which indigenous people live on the margins of society after their displacement by settlers. Since this phrase was coined in the 1970s, aboriginal people in Australia and Native American tribes in the United States have been considered fourth-world groups. Fourth-world people are among the poorest in the world.