Two entities are sometimes referred to as an Oracle database—the instance and the database—and people often confuse them.
In the Oracle world, the term database refers to the physical storage of information, while instance refers to the software executing on the server that provides access to the information in the database. The instance runs on the computer or server; the database is stored on the disks attached to the server, as shown in Figure 1-1.
The database is physical: it consists of files stored on disks. The instance is logical: it consists of in-memory structures and processes on the server. An instance can connect to one and only one database. Instances are temporal, but databases, with proper maintenance, last forever.
Users do not directly access the information in an Oracle database. Instead, they pass requests for information to an Oracle instance.
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