A database link is a schema object in one database that enables you to access objects on another database. The other database need not be an Oracle Database system. However, to access non-Oracle systems you must use Oracle Heterogeneous Services.
A database link (same as linked servers in SQL Server) is an object in one Oracle database that defines a path to another database. They allow you to query remote tables and execute remote procedures residing in an Oracle User on that remote Oracle database. That way, you can use tables and other objects from that database in a SQL statement that you execute locally. Say, for example, that you had a table called USER_MASTER in a remote database (ABC), and that you wanted to access that data while connected to your local database (XYZ) and based on the information that you get from that table, you need to decide what your processing logic is going to be.
A database link is what you need. Before creating it, you must collect the following information:
a) A net service name that your local database instance can use to connect to the remote instance, and
b) A valid username and password on the remote database.
The net service name is necessary for every database link. The username and password that you specify when defining a database link are used to establish the connection to the remote instance.