Short answers, pulled from the story.
The Second World emerged in 1961 as one of three global groupings during a tense period. It gathered states aligned with the Eastern Bloc and the Soviet Union to stand directly opposed to the First World.
Yugoslavia and China serve as prime examples of nations that eventually broke away from Soviet ideology while keeping their communist governments intact. These divergent paths challenged the notion of a monolithic bloc within the Second World grouping.
Most communist states lost their alignment with Moscow after the upheavals of 1989 which marked the collapse of Soviet influence in the region. The dissolution of the Soviet Union occurred on the 26th of December 1991 following these changes.
Five communist states remained independent after the Soviet collapse in 1991. China, Cuba, Laos, North Korea, and Vietnam continued as communist governments into the modern era.
The meaning changed because sociologists began using developed, developing, and underdeveloped as replacement terms for global stratification. Modern usage describes regions based on economic entities rather than political alignment.