The Third World used to mean all of the nations that were not aligned with either the United States or the godless communists. It eventually became a catch-all term for the less developed parts of the world.
More recently, the term "developing world" has generally been preferred, but with the vast socioeconomic differences that have arisen even now it is considered out-of-date. Many people now break countries up into more new categories, such as economically-developed countries (EDCs), newly-industrializing countries (NICs), lesser-developed countries (LDCs), and least-developed countries (LLDCs).
If you are reading this, you are probably fortunate enough to live in an EDC.