Bitcoin is entirely popular people to believe it, so the monetary system is unstable, finally battle between faith and right!
Currency is a representation of money. One dollar bill represents 1 USD, 10 dollar bill represents 10 USD.
Money are financial instruments. Dollar is a unit of account for Federal Reserve notes which is issued by US govt as promissory of "legal tender". Meaning the govt will accept is as payment of debt. i.e. taxes. That's what makes it money