The terms first second and third world became commonly used during the cold war era. At that time the "first world" referred to the United States and its allies. The term "second world" referred to the communist bloc countries (although this term was seldom used) and the term "third world referred to those countries which were not aligned with the capitalist or communist powers.

Since the collapse of the Soviet Union, the term 'second world' ceased to be used, The term 'first world' referred to all developed nations and 'third world' referred to countries with poor infrastructure. Third world countries typically have high birth rates, low life expectency low per capita GDPand a low level of development on the human development index. Some analysts also use the term 'fourth world' to refer to the very poorest of the third world countries.

One clear explanation of 'geopolitics' that I found is by Greg Cruey at: http://goasia.about.com/od/countriesaz/a/asiadefined.htm, where he says: "Geopolitical units have replaced continents in the public mind. The Middle East, for example, extending from Iran through the Arabian Peninsula and across North Africa, is not a continent. Algeria is in the Middle East, and on the continent of Africa. Iran is also in the Middle East, but on the continent of Asia. The middle East is not unique in this; terms like "Central America" and "Latin America" are both intended to replace geographic categories by using criteria other than just geography. And until after WWII, Southeast Asia was an unthought of idea: Burma and Thailand were "Greater India" and Vietnam and Cambodia were Indo-China. Geography may not change much, but time and politics seem to change how we interpret it."