From what most people define, when we talk about currency, this is a generally accepted form of money, this can be paper notes, coins, which is naturally issued by the government and then circulated in the economy.
Cryptocurrency on the other hand is not that different from tokens. These tokens are issued by enthusiastic developers and someone in the community who bought them (or received it for something in return). They can be traded, and when that occurs, they become a cryptocurrency.
But, these tokens are more than just a cryptocurrency. They can be used in some cases in the operation of smart contracts in the network that accepts the specific token. Let us say for instance, BAT or the Basic Attention Tokens are used in an advertising related service, like they may be with the GToken. This means that BAT value is useful for those who want to increase their engagement in golf, so they will buy this type of token.
So cryptocurrencies help the transfer of value in the community that accepts it and in some cases, functions as money does. And, in some cases, this token can be used in the blockchain, as the GToken will be used.