The statistical databases organize countries by analytical criteria. The country classification divides the world into two major groups: Advanced Economies and Emerging and Developing Economies. This classification is not based on strict criteria, economic or otherwise, but instead has evolved over time with the objective of facilitating analysis by providing a reasonably meaningful organization of the data.
You can view the current groups' composition in line with the World Economic Outlook (WEO) here. Please also see the methodological notes on the various products' portals.