A Private Synonyms can be accessed only by the owner.

Public synonyms can be accessed by any user on the database.

An Index is an optional structure associated with a table to have direct access to rows, which can be created to increase the performance of data retrieval. Index can be created on one or more columns of a table.

A Cluster with an index on the Cluster Key.

Hash clusters are better choice when a table is often queried with equality queries. For such queries the specified cluster key value is hashed. The resulting hash key value points directly to the area on disk that stores the specified rows.

Private Database Link, Public Database Link & Network Database Link.

Network database link is created and managed by a network domain service. A network database link can be used when any user of any database in the network specifies a global object name in a SQL statement or object definition.