Oracle database usually contains one database and a single instance. But Oracle 9i 10g RAC (Real Application Clusters) can have multiple instances to interact with a single database for high availability.

Instance is non-persistent memory based background processes and structures.

Database is persistent disk based data and control files

Physical Structure of Database:

One or more datafiles control file(s) Redolog file(s) and init.ora file

Logical Structure of Database:

Tablespaces segments extents blocks

ther is Physical structure & Logical structure

Physical structure Consits of 3 files
one or more data files
two or more redo lof giles
one or more control files

Logical Structure includes

Tablespaces - is database divides into logical storage units
database scheme objects - logical structures that refer to database data