The rich and the poor

Hello people this is my first post, good day to you.

I like to know how do the ecomomic system actually works in the simplest term street person like me can grasp in comparing the two or more countries.

What actually make one country richer than the other? how do we know the accuracy of the data and who decide that this side has certain amount of wealth compare to other in such precission?

Thanks very much in advance for your informations.