Try the below links.
In 11g the standby database is also called active standby database, coz you can read the database(ie, you can query using select statement) and at same time log shipping also will occur. Flashback database feature is must and should be enabled at both primary and standby databases, So that you can open the standby database in read/write mode which is called snapshot standby database. The reason is , when you open the standby database in read/write mode, it will create a snapshot(restore point) at that state using the flashback what you have enabled. when the snapshot is again converted to active standby (read/log apply), the standby database will be restored using the restore point what its create at the time snapshot standby. So both Active standby and flashback database feature tightly coupled to give us a great option to work with.
Hope this helps.