So the 'North' is used sometimes to mean rich and developed countries.

And the 'South' is used to mean poorer and less developed countries.

When people use these expressions, a place like Australia - although it's in the Southern Hemisphere - is considered as part of 'the North'.

The 'North'    The 'South'

Related words:

The DEVELOPED COUNTRIES are the industrialized nations.
The DEVELOPING COUNTRIES are poorer countries. Usually there is a low level of technological development and a low standard of living.

The THIRD WORLD: the poor, developing countries of the world.
The FIRST WORLD includes the US, Canada, Western Europe, Japan, Australia and New Zealand. (This expression is not often used)

Majority World Countries are the poorer and less developed countries where most people of the world live.

Which countries are developed and which are developing can be different depending on what criteria are applied.

Usually in a developed country the average income per person in over $10,000. Usually in a developing country the income per person is less than $5,000. (Obviously there are some countries in between these amounts.