Oracle User, Schema, and Database

USER and SCHEMA

There is a one-to-one correspondence between a user and a schema.

The user owns the schema.  But, without privileges, the user can not do ANYTHING.

Once granted with proper privileges, the user can create OBJECTS in the owned schema where database is one of the objects.

A user may be granted to gain access to other schemas and the objects belonging to those schemas.

Using roles makes granting of privileges easier.

A role is equivalent of certain privileges by design from the DBA or any user (if this user is granted with the creation of roles).

INSTANCE and DATABASE

It is difficult to create multiple databases within one instance.   Normally when you create an instance, you create a database with the same name as the instance.

But, one can create different users under this database/instance.  Each user will have own space, tables, access control, and the tables can only be viewed/updated by other users with privileges granted to them by database owner explicitly.