Blockchain and ledger are two related concepts that are often confused with one another. A blockchain is a digital ledger that records transactions in a public, decentralized network. It uses cryptography to create an immutable record of all transactions ever made within the network, which means that there's no way for anyone—including the owner of the blockchain—to change anything on it.
On the other hand, a blockchain is not necessarily a ledger. A ledger is simply an accounting book that keeps track of financial transactions in an organization or business. Any system can be used as a ledger—even if it's not built to use cryptography or be decentralized.
The difference between these two terms becomes especially important when discussing cryptocurrency, especially because they're often used interchangeably in conversations about Bitcoin and Ethereum (the two most popular cryptocurrencies).