I'm lovin' it! According to the Human Develpment Index or HDI, Norway is the most developed country based on economic and social factors such as GDP, Literacy Rate, Amount of Education, and Life Expectancy. The Democratic Republic of the Congo, on the other hand, is at the bottom of the Index. The US sits around 8th place and is considered one of the most developed countries.
When talking about the wealth of a country, we generally bring up GDP or GDP per capita. This is the total sum of all goods and services sold by companies within a country's borders usually within a year. When we use the "per capita" designation, it refers to the GDP divided among all the citizens of the country or average amount of GDP per person. This number can be misleading since many LDCs do not properly distribute wealth and therefore there are a few very wealthy people and most people with nothing.
We may also refer to the "Big Mac" index which is a type of tool to look at PPP or Purchasing Power Parity. Basically if you bought a product in America, could you exchange your money for another country's currency and purchase that same product with the money you received in the exchange. You can find the Big Mac index here and it explains a little more about it.