Cryptocurrency
In many cases, it is not possible to exchange cryptocurrencies for real currencies such as the US dollar. It is only possible to convert them to other cryptocurrencies. People can also use them to buy things. But you can convert some cryptocurrencies to real currencies. Those cryptocurrencies usually have high volatility, (their price changes very often).  Using them is very risky. They are also a target for Pump-and-Dump-Attacks. They act like a big distributed economic system: because they are not issued or controlled by central banks, their value is difficult to influence.  For this reason, they cannot really take the place of a stable currency.