A useful way to hold this phrase is to picture a Cold War map with the countries sorted into numbered groups: one, the side that stood with the United States and Western Europe; two, the Soviet side; three, everyone who joined neither. The numbers were about which side a country was on. Because most of the rich industrial states happened to sit in group one, people started using the First World to mean simply the wealthy countries — and that is the meaning you meet far more often today, usually with a poorer group set against it.
可以想成一張冷戰時期的地圖，上面的國家被編成幾組：一號，站在美國和西歐這一邊；二號，蘇聯那一邊；三號，兩邊都不站的。這些編號本來只是在標「你站哪一邊」。因為富裕的工業國家剛好大多落在一號那一組，久了大家就直接拿 the First World 來指「有錢的國家」。今天看到這個詞，多半是後面這個意思，而且旁邊通常還有一群比較窮的國家在對照。