A distributed database is a database in which portions of the database are stored in multiple physical locations and processing is distributed among multiple database nodes.
A centralized distributed database management system (DDBMS) integrates the data logically so it can be managed as if it were all stored in the same location. The DDBMS synchronizes all the data periodically and ensures that updates and deletes performed on the data at one location will be automatically reflected in the data stored elsewhere.
Distributed databases can be homogenous or heterogeneous. In a homeogenous distributed database system, all the physical locations have the same underlying hardware and run the same operating systems and database applications. In a heterogeneous distributed database, the hardware, operating systems or database applications may be different at each of the locations.