A synonym is an alias for a table (or a view). Users can execute SQL statements against the synonym, and the database will map them into statements against the object to which the synonym points.
Private synonyms are schema objects. Either they must be in your own schema, or they must be qualified with the schema name. Public synonyms exist independently of a schema. A public synonym can be referred to by any user to whom permission has been granted to see it without the need to qualify it with a schema name.
Private synonyms must be a unique name within their schema. Public synonyms can have the same name as schema objects. When executing statements that address objects without a schema qualifier, Oracle will first look for the object in the local schema, and only if it cannot be found will it look for a public synonym.