Oracle Database Concepts - Oracle 10g Database Administration Concepts and Implementation Made Simple - page 92

4) Figure 3-8: Redo log files' utilization in action. Redo log
files get utilized in a circular fashion. A database should
have at least two Redo Log groups. Each Redo Log
group should have two or more redo log files called
“members”. Redo Log files get utilized by Oracle
Instance in a circular fashion. Redo Log files and
Archived Redo Log files play a great role in Oracle
Database recovery in times of any kind of failure.