in many cases, cryptocurrencies cannot be converted to real currencies; it is only possible to convert them to other cryptocurrencies, or to use them to buy things. some cryptocurrencies can be converted to real currencies: they usually have a high volatility, and using them carries a high risk. they are also a target for so-called pump-and-dump-attacks. they act like a big distributed economic system: as 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.