Rich and poor in civilization can for instance be if you have a lot of technology and resources in your country. Say that you are the British Empire with a lot of technology and resources, which makes you a wealthy country in civilization, then you would like to cooperate with a rich country like the American empire, not a poor country like the Mongolian empire who often have few resources and very little technology. In the real world you can compare this to Norway who have a lot of resources and is a wealthy country. They want to cooperate with other countries with high degree of competence that can help to enhance the Norwegian economy and provide them with new knowledge to exploit their resources as much as possible. It is as they say, “Birds of a feather flock together”.
In civilization there is also slavery. Slavery is in a way a symbol of the differences between rich and poor. In the earlier years there was a lot of slavery in the world. Many of them was forced away from their homes in Africa and taken to USA and Europe where they were sold as slaves. Step by step, more and more countries outlawed slavery. USA was one of the countries with most slavery in the 18th century. The slaves had no rights and were often sold and bought. They didn´t own anything and they had no money. They didn´t even get paid. In the end this ended in the American Civil war. In civilization you can choose to introduce slavery in your country. If you do so you will gain economical advantage, but it might be riots in your country and the people won´t be happy.
A limitation in the game relative to the real world is for example when another country wants to cooperate with you. What´s happening then is that the proposal from the other country appears on your screen and you can only either accept or decline. Say the proposal is open borders with the American Empire and you choose to accept it. From the moment you click accept, the Americans will be right next to you and you have open borders with them – easy as a pie. In the real world I´m afraid it isn’t just as easy if someone comes with a proposal to cooperate with your country. First the government has to agree with it, after that I´m pretty sure your allies has something to say, and finally you have to go trough tons of paperwork.