A database server is a computer that stores DBMS (database management system, for example, Oracle, Sybase) software, and a database shared by a network of computer clients.
Most databases are used in a client/server environment. By storing data on a single, powerful machine on a network (server), the data is centralized and accessible to many users (clients).
The server ensures that the data is maintained correctly and serves as a traffic cop to regulate clients machine access to the data. The server's computing power is also useful in computing and filtering data from the database before it consumes network resources and reaches your workstation. For small or localized databases, your own computer may act as both database server and client.