This thread is for the significance of the term, what it applies to, history, where it came from, its use in philosophy etc...
What is a 1st world country?
The 1st-2nd-3rd world dichotomy comes from the Cold War. The industrial (formerly colonial, in many cases) powers which were not communist generally aligned themselves with the United States against the 2nd World, which was led by the Soviet Union. The 2nd World, of course, comprised the communist nations, though a few of these, such as China and Yugoslavia, ultimately parted ways with the Soviets after a little while (though it should be noted that China is often considered 3rd World instead of 2nd World). The 3rd World tried to stay out of the fight, though in practice most of these were closer to the 2nd World than the 1st World, often embracing Socialist politics.
Ironically, the 2nd World ended up better off than the 3rd World in most cases, even though you'd think that full-blown communism would prove more disastrous than a form of socialism that allowed some private enterprise to continue. Their economic system worked better for whatever reason and of course they were less susceptible to foreign meddling and/or civil war that destabilized economies and set back years of progress, since they fell under the umbrella of Soviet protection.
Today, the 2nd World is increasingly irrelevant, making it far more of a 1st-3rd world dichotomy than anything else.
Two countries which once could've been considered part of the 2nd World but today are decisively 3rd World include Ethiopia and Afghanistan. Likewise, two countries today that are not officially communist but which are considered to be 2nd World holdouts include Angola and Belarus.
Most 3rd world nations are run by warlords, not communism.