Each database object has a name. You can use a quoted or nonquoted identifier to indicate the name of an object in an SQL statement.
In fact, you can use a quoted or nonquoted identifier to name any database object.
A quoted identifier begins and ends with double quotation marks ("). If you use a quoted identifier to name a schema object, you must add the double quotation marks when you reference the object.
A nonquoted identifier carries no punctuation mark.
Database names, global database names, and database link names are case-insensitive and stored in uppercase. If you specify a quoted identifier for a name of this type, the quotation marks are silently ignored. For more information about how to name a user, see CREATE USER.