Third World Countries?

"Third World" has been boggling my mind for a few months. I sought the reason behind this term but not yet able to have any qualified reasons. Is this a reality or just an ideology? What is the measuring scale of such stereotyping. Maybe it's a different socio-economic class or maybe it's their first time-around or maybe still baffled with cold war.

Don't we need to change this pocket-size political orientation?


PS: Image source is wikipedia. (http://upload.wikimedia.org/wikipedia/commons/thumb/0/08/First_second_third_worlds_map.svg/800px-First_second_third_worlds_map.svg.png)
- Countries who claim to be first world are in blue.
- Countries who claim to be second world are in red.
- Countries who claim to be third world are in green.