Oracle Instance Concepts - Oracle 10g Database Administration Concepts and Implementation Made Simple - page 93

Chapter 4:
Oracle Instance Concepts
What is Oracle Instance?
Oracle system or more commonly we use the word Oracle server
or Oracle Database, whatever name you use, just keep one thing
in mind that Oracle has two pieces. Once is called Oracle
Instance and the other one is Oracle Database. User sends
commands to the Oracle Instance (Genie) and Oracle Instance
after interacting with the database will send the result back to the
user.
In this chapter we will only talk about Oracle Instance and you
can visualize it as a genie. You cannot retrieve the data directly
from the database. You have to send the request to Oracle
Instance and it can fulfill your request.