USA dollars is the physical money in an economy, comprising the coins and paper notes in circulation. Currency makes up just a small amount of the overall money supply.
While the economy's performance is at the heart of the decision to buy or sell dollars. A strong economy will attract investment from all over the world due to the perceived safety and the ability to achieve an acceptable rate of return on investment. So we all see that currency and economy are not the same. Using USA dollar and economy as a good example.