A database running on the World Wide Web is most often using a client-server network architecture.
A user (client) with permissions associated with their account can change entries that are stored on a centralized server. By changing the ‘master copy’, whenever a user accesses a database using their computer, they will get the updated version of the database entry. Control of the database remains with administrators, allowing for access and permissions to be maintained be a central authority.
For a blockchain database, each participant maintains, calculates and updates new entries into the database. All nodes work together to ensure they are all coming to the same conclusions, providing in-built security for the network.
The consequences of this difference is that blockchains are well-suited as a system of record for certain functions, while a centralized database is entirely appropriate for other functions.