And I need to emphasize that Bitcoin is a ledger first, and a currency second...Bitcoin is above all a database. What is this database used for? It is used for keep accounts. What kind of accounts does it keep? It records the transaction and transfer records of Bitcoin between different people. This is what Bitcoin does. It exists as an exchange equivalent, as a medium of exchange. When we all regard Bitcoin as the general equivalent in our transaction process, it plays the role of currency. Then it is considered a currency.
Bitcoin is a currency and the technology behind it is the
blockchain. Both terms are being used interchangeably although they don't refer to the same thing.
The blockchain technology is not limited to bitcoin (or other currencies) and it has many different applications. Since it's decentralized then it would be better to compare it to a destributed database than a regular/centralized database.