To add to Eleanor's comments, "third world" was coined during the 1950s. It is now largely out of fashion for two reasons. First, because the "Cold War" is over.
Second, because what was once a useful description is now seen as being disparaging.
In the world of investing, for example, these countries are not called "Third World" countries nor "developing nations"--they are called "emerging markets."
Sometime the phrase 'third world country' is used to refer to poor or developing countries. However, this is a misconception, it does not mean a poor country. During the Cold war, the countries which were on America's side were called 'first world countries' and the countries that were on Russia's side were called 'second world countries'. Third world countries are countries that were not on either side, for example African countries.
Most of the time, though, people use the phrase when talking about developing countries.
"Over the last few decades since the fall of the Soviet Union and the end of the Cold War, the term Third World has been used interchangeably with the least developed countries, Global South and developing countries to describe poorer countries that have struggled to attain steady economic development, a term that often includes "Second World" countries like Laos. This usage, however, has become less preferred in recent years."----read more @ http://en.wikipedia.org/wiki/Third_World
"Due to the complex history of evolving meanings and contexts, there is no clear or agreed upon definition of the Third World. Some countries in the Communist Bloc, such as Cuba, were often regarded as "Third World". Because many Third World countries were extremely poor, and non-industrialized, it became a stereotype to refer to poor countries as "third world countries", yet the "Third World" term is also often taken to include newly industrialized countries like Brazil or China. Historically, some European countries were part of the non-aligned movement and a few were and are very prosperous, including Austria, Ireland and Switzerland."----read more @
"The term Third World arose during the Cold War to define countries that remained non-aligned with either NATO (with the United States, Western European nations and their allies representing the First World), or the Communist Bloc (with the Soviet Union, China, Cuba, and their allies representing the Second World). This terminology provided a way of broadly categorizing the nations of the Earth into three groups based on social, political, cultural and economic divisions. The Third World was normally seen to include many countries with colonial pasts in Africa, Latin America, Oceania and Asia. It was also sometimes taken as synonymous with countries in the Non-Aligned Movement. In the so-called dependency theory of thinkers like Raul Prebisch, Walter Rodney, Theotonio dos Santos, and Andre Gunder Frank, the Third World has also been connected to the world economic division as "periphery" countries in the world system that is dominated by the "core" countries."---read more @ http://en.wikipedia.org/wiki/Third_World