In English, world is rooted in a compound of the obsolete words were, man, and eld, age; thus, its oldest meaning is "age or life of man". Its primary modern meaning is the planet Earth, especially when capitalized: the World. In this sense, a world map is a map of the surface of the Earth. World can also refer to human population in general or to a distinct group of people

The terms First World, Second World, and Third World were used to divide the nations of Earth into three broad categories.  

The three terms did not arise simultaneously. After World War II it became common to speak of the capitalist and Communist countries as two major blocs, scarcely using such terms as the "free world" as compared to the "communist bloc". The two "worlds" were not numbered. It was eventually pointed out that there were a great many countries that fit into neither category, and in the 1950s this latter 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." These latter terms were always much less common.

In the context of the Cold War:
First World refers to nations that were within the Western European and United States' sphere of influence — e.g., the NATO countries of North America and Western Europe, Japan, South Korea, and some of the former British colonies such as Australia, New Zealand, and South Africa.
Second World referred to nations within the Soviet Union's sphere of influence, principally the Warsaw Pact countries. Besides the Soviet Union proper, most of Eastern Europe was run by satellite governments working closely with Moscow. This term may or may not also refer to Communist countries whose leadership were at odds with Moscow, e.g. China and Yugoslavia. Recently, this term has been used to describe former Third World countries that have experienced too much development to be classified any longer as being a part of the Third World (such as Costa Rica).
Third World refers to nations within neither sphere of influence, who were often members of the Non-Aligned Movement. They were mostly developing countries, and many of them are located in Africa, Latin America, and Asia. They are often nations that were colonized by another nation in the past. After World War II, the First and Second Worlds struggled to expand their respective spheres of influence to the Third World. The militaries and intelligence services of the United States and the Soviet Union worked both secretly and overtly to influence Third World governments, with mixed success.

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.

With the 1991 collapse of the Soviet Union, the term "Second World" largely fell out of use, though the term "Third World" remains popular, mostly as another term for developing countries. The remaining Communist countries either became more isolated from the world economy, as in North Korea and Cuba, or began integrating capitalist concepts such as private enterprise into their societies and forging new trading ties with external capitalist economies, as in Vietnam and China.

In more recent use, the term First World refers to developed nations, while Third World, in contrast, refers to developing/undeveloped nations.