what is the oracle instance?

Instance is a combination of memory structure and process structure. Memory structure is SGA and Process structure is background processes.

SGA - System or Shared Global Area.

Components of SGA:-

DBBC - Database Buffer Cache

SP - Shared Pool. It is futher divided into Library Cache(LC) and Data Dictionary Cache(DDC) or Row Cache.

RLB - Redolog Buffer

Background Process:-

1. Mandatory Processes-

a. SMON - System Monitor

b. PMON - Process Monitor

c. DBWR - Database writer

d. LGWR - Log Writer

e. CKPT - Check point

f. RECO - Recoverer

2. Optional Process:-

a. ARCN

b. RBAC

c. MMAN

d. MMON

e. MMNL