Each country has a currency and its own economic system, which are affected by the exchange rate fluctuation. Purchasing power parity (PPP) is a way of measuring the purchasing power of countries in the same currency to map the difference in the cost of living in each nation. For example, PPC measures how much a dollar purchases of an X product in different countries.
On the other hand, GDP measures the entire production of goods and services of a nation in a given period. Nations with higher GNP in comparison to others will be enriching and raising their standard of living more quickly than others.
Thus, both GDP and PPP indicators are ways of measuring the relative wealth and relative standard of living of nations.