What are 1st, 2nd and 3rd world countries? What does this mean?

First World- indusrialized/developed market-economy nations like the U.S., Western European nations, Japan.

Second World- planned economies like Russia, Poland, and now perhaps China.

Third World- originally neither US nor USSR bloc counties; now developing countries like African and Latin American nations.

http://www.undergroundtravel.com/travel-savvy/world-divisions/world-divisions.htm

I once heard a reference to Haiti as a "4th World" country, defined as declining/decaying socially and politically.