In geopolitics, the Global South is a general term that roughly designates ex-colonies and imperialised countries.
In contrast, the Global North are the most advanced capitalist economies, meaning the ones that benefit from imperialism.
It's important to note that as this is a geopolitical term, some leeway must be accounted for. For example, China is generally considered by financial institutions to be part of the South, despite them soon achieving the highest GDP in the world. Israel is considered to be part of the Global North by the World Bank, due to their strategic value in the Middle East.
The term Third-World is a disambiguation; it originally referred to "non-aligned" countries during the Cold War, and mostly (if not only) in the "first-world" aligned nations (the first world being the bloc aligned with the USA).
The term "developing economies" is also misleading and disrespectful; it's not that they are developing but can't develop, as their imperial masters will not allow it.
For these reasons, when talking about the imperialised and colonized countries, it is preferable to use the term Global South.