Schema objects are the logical structures that directly refer to the database's data. Schema objects include tables, views, sequences, synonyms, indexes, clusters, database triggers, procedures, functions
There are two types of Synonyms Private and Public.
A Private Synonyms can be accessed only by the owner.
Public synonyms can be accessed by any user on the database.
Synonyms are used to : Mask the real name and owner of an object.Provide public access to an objectProvide location transparency for tables, views or program units of a remote database.Simplify the SQL
Private Database Link, Public Database Link & Network Database Link.
Private database link is created on behalf of a specific user. A private database link can be used only when the owner of the link specifies a global object name in a SQL statement or in the definition