What is the third world?

During the Cold War and era of the Soviet Union, the term third world refered to those countries that were unaligned with either the Soviet Union or the United States. These third world countries were often the less-developed countries of the world.
The first world refered to the United States and its allies while the second world refered to the USSR and its allies.
Today, these "worlds" are obsolete so most refer to two groups of countries as being either "developed" versus "less-developed" or "developing."