The terms First World, Second World and Third World can be used to divide the nations of Earth into three broad categories. The term Second World has largely fallen out of use because the circumstances to which it referred largely ended with the 1991 collapse of the Soviet Union.

The three terms did not arise simultaneously. After World War II, people began to speak of the NATO and Warsaw Pact countries as two major blocs, often using such terms as the "free world" as compared to the "Soviet bloc". It was eventually pointed out that there were a great many countries that fit into neither category, and in the 1950s this group came to be called the Third World. It then began to seem that there ought to be a "First World" and a "Second World".

Eventually, it became common practice to refer to nations within the Soviet Union's sphere of influence (e.g. the Warsaw Pact countries) as the Second World. Besides the Soviet Union proper, most of Eastern Europe was run by satellite governments working closely with Moscow. The term "Second world" may or may not also refer to Communist countries whose leadership were at odds with Moscow, e.g. China and Yugoslavia.

There were a number of countries which did not fit comfortably into this neat definition of partition, including Switzerland, Sweden, and the Republic of Ireland, which chose to be neutral. Finland was under the Soviet Union's sphere of influence but was not communist, nor was it a member of the Warsaw Pact. Austria was under the United States' sphere of influence, but in 1955, when the country again became a fully independent republic, it did so under the condition that it remained neutral. Yugoslavia, a communist east European country, was a founder member of the Non-Aligned Movement. Albania was a communist east European country which withdrew from the Warsaw Pact over ideological differences in 1968 and had stopped supporting the pact as early as 1962.

In recent years, as many "developing" countries have industrialized, the term Fourth World has been coined to refer to countries that have been left out of economic globalization and still lack industrial infrastructure.