These are the pure digital currency created using cryptography and works on blockchain mechanism .
There is no centralized system involved in cryptocurrency transactions, like in case of traditional physical currency system where there is a centralized system (like bank/payment gateway) but in cryptocurrency there is no centralized system/server involved ,obviously no middleman involved and the transaction using cryptocurrency takes place between only 2 parties the sender and the receiver .
But yes the transactions which takes place using cryptocurrency are tracable , each transaction lefts a log which is called block chain .
Unlike credit card/debit card transactions , no one can hack/misuse crypto currency., or no one can repeat the same transaction as the cryptocurrency transactions are secured via public key and private key combination .