Concepts such as "distributed" and "decentralized" apply to the structure and maintenance of the ledger. To understand the difference, consider examples of centralized registers, such as government records of real estate sales, ATM banking transactions, or lists of goods sold on marketplaces. In these cases, the registry is managed by a single organization: a government agency, a bank, or a marketplace. There is only one main copy of the registry, and all others are simply backup copies with no official status. Therefore, traditional registries are centralized because they are managed by a single organization and depend on a single database.
In contrast, blockchain is a distributed system that functions as a decentralized ledger. There is no single copy (distributed storage is used instead) and no single controlling authority (decentralization). Simply put, each participant who decides to join and participate in the blockchain network stores an electronic copy of all data, which is regularly updated with each new transaction, similar to other participants.