what is clusters in oracle ?

The term cluster can refer to either:

* An Oracle object that allows one to store related rows from different tables in the same data block. Table clustering is very seldomly used by Oracle DBA's and Developers.

* Two or more computers that share resources and work together to form a larger logical computing unit. RAC and Oracle Parallel Server can be used to access Oracle from multiple nodes of a clustered system.

In database computing, Oracle Real Application Clusters (RAC) is an option for the Oracle Database software produced by Oracle Corporation and introduced in 2001 with Oracle9i — provides software for clustering and high availability in Oracle database environments. RAC is included with the Standard Edition of Oracle Database, but it is an extra-charge option for the Enterprise Edition
Oracle RAC allows multiple computers to run Oracle RDBMS software simultaneously while accessing a single database, thus providing a clustered database.
In a non-RAC Oracle database, a single instance accesses a single database. Where the "database" consists of a collection of data files, control files, and redo logs located on disk; the "instance" comprises the collection of Oracle-related memory and operating system processes that run on a computer system.
In an Oracle RAC environment, two or more computers (each with an instance) concurrently access a single database. This allows an application or user to connect to either computer and have access to a single coordinated set of data