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...Oracle Architecture
An instance of the database allows access to the database files
Notes:
A picture is worth a thousand words. Here we see that in order to access the database, we must have an instance started.
Note that an instance provides access to one database (plus other databases via Distributed Database). However, a single database can be accessed by more than one instance; this is accomplished by using the Oracle Parallel Server (OPS) architecture in Oracle releases prior to Oracle9i and is accomplished by using Real Application Clusters (RAC) in Oracle9i and later.
Note also that a single computer can run more than one instance, or more than one database/instance. (A single machine running more than one instance is typically a unit testing computer in the development environment. A production database/instance normally runs on a dedicated machine.)