From release 9i onward, an Oracle database can be configured so that nomatter what happens, the database will never lose a single row ofcommitted data. It is also possible to configure an environment for onehundred percent availability. This ideal configuration requires use ofData Guard and RAC (or possibly Streams). A single-instance,non-distributed environment cannot achieve this—but it can get veryclose.

Please have Chapter 15 of the book"OCAOracle Database 11g: Administration I Exam Guide"by John Watson,copyright 2008 from Oracle Press, a division of McGraw-Hill. atattachment.

This chapter will go through the concepts behind Oracle'sbackup and recovery mechanisms: the enabling structure within which youwill configure whatever level of data security and availability isdemanded by your organization. The next two chapters will cover thepracticalities of backup, restore, and recovery. But always be awarethat this is a superficial treatment; the second OCP examination dealswith backup and recovery in much greater detail. And even after passingthat, you will not necessarily be fully competent. This is an areawhere you cannot do enough studying, research, and (most importantly)practice.

Recommended for reading.