Replication is the ability of a database to create copies of a master database at remote sites. These copies are sometimes called snapshots and may contain the whole database or any subcomponent of the database.
For example, in a relational database, a CUSTOMER table may be snapped to many remote sites for read-only query. Subsets of the CUSTOMER table may be specified, requesting only specific rows and columns, and these replications are refreshed on a periodic basis.

The Publish/Subscribe Model for Oracle Replication is a more flexible method for replication than was the Oracle7 snapshot replication.
Oracle advanced queuing has initiated a new replication model dubbed "publish/subscribe" that differ from the traditional replication model in a very important way.

The sender of the replicated data (called the "publisher"), does not have to know anything about what's-up on the receiver-side (called the "subscriber"). With replication independence we no longer have to complex feed and verify procedures and the dbms_aq package allows some interesting replication functions: listen Procedure The new dbms_aq.listen procedure allows you to "listen" for incoming messages (rows), just as-if you were the registered subscriber. The listen procedure has a newer input parm method, agent_list) which is a Oracle abstract data type (ADT):

agent_list IN AQ$_AGENT_LIST_T,
wait IN BINARY_INTEGER DEFAULT DBMS_AQ.FOREVER, 
agent OUT SYS.AQ$_AGENT);